We tend to regard the will as something that is marked off from the rest of the person, because, somehow, it is a direct manifestation of the person's being. So an ADHD drug could not be described as "strengthening people's will", because it if were described in this way it could not then be said to be be their own will that was being strengthened; they would be having it down for them.
Similarly, one might think, you can do my work for me, but not my thinking, because then it would not be my thinking that was being done. (Still, in that sense you would not be doing my work - my working - and it is just as impossible for you to do my work as it is for you to think my thoughts or even perhaps to wear my boots, taken to be the ones I am wearing ("Look, his boots (borrowed boots) have mud on them")).
One might on the other hand regard the will as the energy or strength to carry something through. Or one might regard it as determination, though here too the paradox shows through. If my determination is bolstered by a drug, is it really determination? But one can surely lack psychic energy or strength for just the sort of reason (anaemia, say) that one lacks physical energy. In such a case, one could well speak of "strengthening a person's will" or ability to carry through.
It seems to me possible by the way that one could learn from a drug, as Peter Kramer seems to imply can happen with Prozac, in his 1993 book Listening to Prozac. So the possibility seems to exist that the drug can actually teach someone who lacks it what strength of will is, and perhaps then they could do more easily "on their own" later.
Your problem is very interesting and difficult.