Assuming you're happy with handwaving and shallow science (and if you're working with wormholes, I think you are), you could simply shove a ton of matter into it. If you think of wormholes as paired spheres, then adding a bunch of matter to one of them could convert it from "wormhole entrance" to "black hole".
My intuitive thinking on this is that wormholes have a tiny entrance through which they stream matter to the other side. Let's further assume that when an object passes through the wormhole, it passes through the threshold at the speed of light - like a sound signal being converted to electrical signal through telephone lines. This is an arbitrary rule, but making everything pass at the same speed avoids weird states in the following calculations.
If the tiny wormhole entrance is, say, 1mm in diameter, then you can use the Schwartzchild radius formula to calculate how much mass the wormhole can contain before light can no longer escape it (6.8E23 kg, or about 1/10 the mass of Earth). If objects moving at the speed of light can't escape the entrance, then it's effectively a black hole.
What happens on the other side is up to you. If gravitational forces don't pass through wormholes, then the other side is simply a wormhole to the center of a black hole. If gravitational forces do pass through wormholes, then the other side becomes a black hole too. You might want to increase the mass required in that case, since you're no longer forcing mass into a 1mm sphere - you're forcing it into a 1mm sphere + transit tube + 1mm sphere on the other end.
So how could this come to be? You somehow need to get that amount of mass into the wormhole. That could be a problem with a visible-scale entrance like the 1mm mentioned above - how could someone shove 10% of the Earth through it? You can fix this partly by adding transit time. Maybe it takes a day for matter to transit through the wormhole. In that case you've got a full day to shove your planet-scale object into it. The other option is to make the wormhole entrance smaller, so it doesn't take that much matter. If you have a "funnel" style wormhole, maybe it has a large mouth but a tiny bottleneck, so fitting a large amount of matter in a small amount of time isn't too hard.
Finally, if you want to "unlock" the wormhole at any point, remember Hawking radiation, which means black holes "boil off" over time. If a black hole is right at the edge of the mass required to be a black hole, then it can be un-blackholed over time via Hawking radiation. You could always introduce a bit of tech-magic that stimulates Hawking radiation to speed up the process. Or even add another (large?) wormhole entrance to the network, increasing the volume of the system, which could also unblackhole the system.