Yes. With taxes. But there are some preconditions and infrastructure required, which is why historically plunder was a superior alternative. On a positive side the preconditions probably fit the image you have for your empire.
Stable and manageable cost of military
This is actually a pretty big issue here. If you have a small peacetime army and waging a war requires large increase in military spending you will almost certainly end up gathering large debt at very high interest rates. This requires having the war pay itself back very fast. Which requires plunder. So you need to get rid of this. Some suggestions follow.
Professional standing army. Increases the costs at peacetime, but means that much of the needed military resources for the war are already ready and available. If wars are fairly regular this saves money since you can buy many supplies at stable prices not at the inflated wartime prices. Also reduces the distruptions of switching to wartime footing to the rest of the economy. Ideally the empire would only start small wars the professional standing army can manage on its ready resources and then rebuild the resources after the war is over. Not letting the army plunder would require paying higher wages, but would improve discipline, which would improve chances of actually winning wars. Probably significantly.
Positive cash flow. Seems obvious, but if you have cash at the treasury, you have less need to borrow money to pay for wars, the costs of warfare will be lower and you have less need for plunder to pay for them. This will also give you a rather decisive advantage when you need credit or supplies, leading to secondary cost benefits. And better supply situation. Which generally is a big help in winning wars.
Short wars. Shorter wars are cheaper, period. Have a clear achievable objective, wage war until it is clear whether you will succeed or not, and then win (or not lose)the peace at the negotiating table. You can't really win a war, victory comes from the treaty that ends the war, so the faster you move to the profitable part of the negotiations the better. This probably fits the "no plunder" empire anyway. You can actually make profit from a war you "lose", if you cut your costs fast enough and your opponent trusts you enough to be willing to pay for you to go away. The "trust" here means they do not suspect you are simply extorting them and will be back next year. And "pay" would include addressing the original reasons for the war.
Efficient is relative here, but if being annexed by your empire causes an economic collapse of the new area, you might as well plunder them. I'd guess free trade within the empire, free movement of people and investment, and some level of a banking system. These would help being in the Empire be an advantage and conserve the value of annexed areas. The empire should also have generally good trade relations with its major neigbours, to prevent annexation from causing a major shift in trade routes.
Taxes should be predictable and collected efficiently. Corrupt tax collectors extorting arbitrary sums of money is not really better than plunder and will cause severe damage to the economy. You want civil servants operating within the law and with oversight, not private actors who bought or inherited the tax collection right. Historically major tax reforms resulted in large budget surpluses - and were started due to major budget deficits - so efficient tax system does give large benefits. In addition to gathering more money, it also causes less damage to the economy. And if you annex a new region, you should get all the tax system upgrade benefits for that region.
Apart from making the wars shorter annexing small regions at a time also decreases the cost of integrating the region to your empire. Annexing only small areas at a time cost less money and gives better results. Asking less than you could also helps in the negotiating table. It makes the opponent more willing to settle fast, lowering the cost of war, and more willing to make other concessions. Such as give you money that will offset your costs. And of course paying your demands will force the enemy to gather the money from its remaining areas on short schedule, which in turn in the future makes him less able to fight you and the people more likely to prefer annexation to the empire. A win-win.
Remember economy in strategy
Trade concessions and trade rights help pay for wars and prepare new regions for annexation. And disagreements about them supply reasons for short, limited wars optimal for low cost expansion.
SJuan76s answer reminded me of this. (The answer started inflating to the point where I just stopped writing and sent it.) In addition to permanent standing army and tax collection by civil servants, you almost certainly need a permanent bureaucracy to run the system. In addition to what is needed to directly support the military and tax collection, the logistics and administration would need to be handled. And the legal system and civil engineering would follow.
So the permanent army needs permanent administration. That administration needs permanent logistics support. The logistics in turn needs roads, harbors, canals and other transportation infrastructure. Which needs separate bureaucracy to handle it. And then you need a separate bureaucracy to do oversight on all that. Maybe they'd even have separate administration for improving administrative efficiency. And another for training bureaucrats. And one for handling promotions. And so on. Bureacracy has a natural tendency to accumulate.
So the empire would probably have quite a lot of bureaucracy.