Giving mana to any class that normally does not use it will display a mana bar, though it will likely not regenerate. Additionally, at least in trinity, all classes can use energy and rage, though they do not show a bar, and rage needs some small core edits to allow other classes to generate it. Runes are infinite and runic power always 0 for non-DK's though that could probably be fixed with some moderate core edits.
All classes actually already have energy and regenerate it on a trinity server, you simply need to make an addon to allow the clients to see it. Something like energy watch might be good to take a look at for that, and just find where to disable the class restriction in its code. Assuming you are on trinity or your core has a similar enough SQL structure, you add mana by adjusting the world.player_classlevelstats that has an entry for base HP and mana for each level for each class, you could also add it via custom equipment, the stat number for it is either 1 or 0. Becareful though, as most spells cost a percentage of your base mana, meaning that somone with 0 base mana casts them for free, and the more base mana you have the worse it actually is.
