It's really easy to do. The Hozan tool is actually probably better and easier than a Morizumi/Phil/whatever for the purpose of just extending the existing threads a little bit, although there's a bit of a learning curve in using the Hozan. It's ~22 rotations of the crank for every cm of threaded spoke, so just figure out by how much you want to extend it and count.
Little trick with the Hozan, I find it easier not to clamp it in a vise, even though it has a mount specifically for that. Instead I use a hand on each side of the tool to help all the rollers engage a spoke, then after that they each help each other move forward on the spoke. Otherwise sometimes you can get stuck rolling in a circle if only one or two rollers engage. I usually cut new spokes to my desired length as a group, then bevel them all with a Dremel as a group. But if I'm just extending or chasing existing threads, I don't even bother to bevel them since the existing thread will help all the rollers engage. And generous amounts of cutting fluid as always.
Yeah, I suppose it doesn't cost much to buy another set of spokes, but there's no guarantee that the person cutting the new set is going to do a better job with the next set than this one.