Six threads is quite a few...
You could do a few things:
1 - Get new nipples: If the current nipples are 12mm you could get some 16mm nipples as they might cover the threads. Some people will say do not do this because the spokes are just too short and they should thread farther into the nipple to prevent possible breakage, so do this at your own risk. Having said that, I have done this before with no problem (although probably only a couple threads were showing with 12mm nipples).
2 - Get new spokes: Use a couple of different spoke calculators and obviously make sure what you buy is longer than what you have.
3 - Get a new hub or rim: Find a new hub or rim with dimensions that can make use of your spokes.
4 - Change the wheel lacing: If you are using a 3 cross you might be able to change it to a 2 cross and have the right length spokes; check with a couple spoke length calculators.
If it was me I would try a 16mm nipple first and see if it was going to cover the threads because that is the easiest, then move on to lacing pattern, then to buying new spokes.