Basically yes. You're probably going to have to do some poking around and testing on your own, but the Java side of a GlobalLootModifier is code like any other class, which means it has access to the vanilla loot functions. Just implement the doApply function and you're set.
If you're clever enough you can make your modifier parse the desired loot function and do a lookup (via the existing registry), rather than only do one thing.
The only reason that loot functions aren't parsed and handled for you (the way conditions are) is because generally a modifier is a loot function (that is, performing the same role), so there wasn't an explicit need.