I can get you started.
open skills.2da and find line 19 (UseMagicDevice). In the AllClassesCanUse column change the 0 to a 1 and save it in your override folder. If you do this, UMD will be a cross-class skill for everyone but assassin, bard or rogue, but they should be able to put points in it. To make it a class skill, you'd have to edit the individual CLS_SKILL_XXXX.2da for each class.
If you're using this in a multiplayer/server setup, you might be able to get away with just having this modified 2da in the server's override folder.