Not to contradict Pierre as he's probably correct in that the holes "should" be filled, but:
Yes, it can be done.....I've done it several times successfully.
The holes in the tangs of gyuto's are quite small, so it's not a huge concern IMHO. You can leave well more then enough of the tang in place to still have a very strong tang. I've always been cautious and ended up with a tang slightly wider than I wanted, but with a gyuto handle (only style of knife I've done) the extra width of tang can be accomodated with the larger handle size. With a petty or paring or something small, this might be a lot bigger of an issue.
The hard part is dealing with the bolster....if said knife has one. The "easy" one's have an affixed bolster...ala the Misono UX-10 knives or the Hiromoto White Steel gyuto's that surfaced a while back which were actually peened on, hollow bolsters. Those were the easiest to convert. If they're welded on bolster, or gawd forbid integral bolsters, it's a huge PITA. That bolster has to be ground off very carefully (heat build up is your mortal enemy) and then you have options. Completely refinish the sides of the blade so that everything matches, or leave the area where the bolster was with a different finish (you'll never match the finishes perfectly). I find the best approach is to try and match the finish of the blade....it won't be perfect, but it's such a small area that it doesn't stand out too much. Blue painters tape will prevent screwing this up.
The tang will have to be shortened quite a bit to fit within a normal wa handle.
Then it's pretty much a normal handle project.
All that said....good luck finding someone to do it.
If priced properly, you're talking about a project that should cost in excess of $300 including a new custom handle.
p.s. I'm not advertising for work as I'd never take on another one of these....they're just no good.
I just figured I have enough experience actually doing them that my input might be valuable.