It probably depends. It might be possible for some types of fusion reactors, but you're really focusing on the wrong problem. Getting tritium or deuterium or whatever fusion fuel you have hot enough to initiate fusion is not hard. Sustaining the reaction and extracting energy from it is the hard part.
As observed in other answers, and basically the same question on the Physics Stack, the limitation of a lens is that it cannot heat anything beyond the temperature of the body it's focusing light from, and the surface of a star is not hot enough for a sustained fusion reaction.
Having said that, one type of fusion we humans have explored (and are still exploring, to my knowledge) is called inertial confinement fusion, and basically uses lasers to generate shockwaves in a fuel pellet, compressing and heating it enough to generate a (brief) fusion reaction.
Inertial confinement fusion (ICF) is a type of fusion energy research that attempts to initiate nuclear fusion reactions by heating and compressing a fuel target, typically in the form of a pellet that most often contains a mixture of deuterium and tritium.
To compress and heat the fuel, energy is delivered to the outer layer of the target using high-energy beams of laser light, electrons or ions, although for a variety of reasons, almost all ICF devices as of 2015 have used lasers. The heated outer layer explodes outward, producing a reaction force against the remainder of the target, accelerating it inwards, compressing the target. This process is designed to create shock waves that travel inward through the target. A sufficiently powerful set of shock waves can compress and heat the fuel at the center so much that fusion reactions occur.
The energy released by these reactions will then heat the surrounding fuel, and if the heating is strong enough this could also begin to undergo fusion. The aim of ICF is to produce a condition known as ignition, where this heating process causes a chain reaction that burns a significant portion of the fuel.
I would suggest that with clever enough materials science and engineering of these fuel pellets, it might be possible to achieve the same thing with an intense burst of focused sunlight, rather than a laser (like we do now), though admittedly, it seems like a long way to go do do it that way rather than using lasers like we do now, and a different technique altogether, like magnetic confinement, is probably a better bet for fusion power as an energy source.
The major technical issue with creating a fusion reaction is not getting up to the necessary temperature, it's sustaining the reaction and extracting energy from it. To that point, at least 5 countries (and possibly as many as 9) have successfully tested fusion reactors so far, though the more common term for them is "hydrogen bombs" (a fission bomb is used to heat and compress a tritium payload, which then creates a more powerful fusion explosion).
So I'd suggest that you're focusing on the wrong problem, as far as fusion goes. Getting your fusion fuel hot enough to create fusion is relatively easy, and we've been able to do that for more than 50 years. The part we haven't pulled off yet is sustaining the reaction in a way we can extract usable energy from, and that's probably where your scientists would have trouble too.