I have a Hozan C-700 spoke threader. I have had done this before when I had an overabundance of spokes that are close to the right length and I’m using double walled rims. It works but you should be careful about how much more you thread the spokes. The 2.0mm portion of DT butted spokes is short. Adding 2 to 5mm of thread is okay but anymore than that starts to get to the where the spoke necks down.
That said, the spoke threader is currently going for $180. I’ve had mine for 30+ years and have used it a few times. I’m not sure I ever got that much value out of it for what it cost. Yes, DT Alpine III are expensive but not
that expensive. They are even cheaper if you buy them from
https://www.rosebikes.com/search?q=alpine+iii]Rose Bikes[/url]. Even with shipping they are economical.