There are numerous technobabbling ways to simulate magics in Trek universes IMHO.
One the first I thought of was DNA manipulation : since in TNG every single DNA alteration change the subject almost instantly, you could for instance imagine some being who would be able to emit some very specialized and targeted viruses toward someone, which would change almost instantly the target's DNA (in a sense it is nothing much more than Borg's assimilating nanites), hence the ability to turn princes (or captains) into frogs. Of course, in game terms, turning a human being into an amphibian (without going to warp 10...) would be a nearly impossible task as best, but you could initiate some minor changes without too much difficulty. The speed at wich the change occurs would also come into accounts.
I don't know if pirokinesy (sp?) is listed somewhere in the RPG extensions, but anyway that's a good way to handling the cast of fireballs (and similarly lighting with electronical manipulation, wind with telekinesy, and so on).
As for metamorphing or invisibility, well, think Founders or Jem'hadars.
More ideas to come... what do you think of those?
"The main difference between Trekkies and Manchester United fans is that Trekkies never trashed a train carriage. So why are the Trekkies the social outcasts?"
Terry Pratchett