Wheel build on to disc/gear hub?

As long as the spoke length is not too different you should be fine. This says it as well as I could so copy and past for this reply.

http://www.gsportbmx.com/ask/index.php#q7

7. I want to swap my rim and keep everything else the same do I need new spokes? Is there an easy way to do it?

      A. In this case you can get away with just a simple trick. Loosen all the spokes in the old wheel till they very nearly come undone entirely. Then just put the new rim alongside the old rim. Be careful to line the valve holes up and make sure the spoke holes are "offset" the same way. Then just switch one spoke at a time until the new rim is in place and the old one is off. Finally just follow the last part of the wheel build guide to get some initial tension in. You can get away with this because all the spokes stay in the same orientation as before so they do not have to take on a radically new lay.