JASS is the advanced way of triggers. The trigger you use are GUI (don't remember what it stands for anymore though, maybe someone else can answer that question). They are actually made by JASS. JASS is the programming language used by Warcraft III. So if you can't do it with JASS, then it is not possible. JASS is the very base, but a lot more complex then GUI triggers. with JASS you really type out the entire command. Some people know this programming language, and others don't bother to learn it. You can get pretty far with GUI triggers, but sometimes (at moments like this) you have to insert a JASS part if GUI triggers don't go that advanced.
But don't think everything is possible with JASS. It still has it's limitations. That's why you will have to ask someone who knows about JASS if it's possible, and if so: Ask him/her to make a JASS command for you. (It is usually common manners to mention the person helping you with big JASS problems in your credits
)
I have browsed some for you and I passed
this thread which has something in common with your problem. And if I may believe Daelin in
this thread, there is something about it in the FAQ.
I think that removing and adding the ability again is probably the best option if there is no JASS for this issue. You just have to make a few more actions after 'Unit - Add Ability' to set the ability level to the same it was when the ability was cast. It's getting too late here, so I'm not bright enough to think of an exact trigger for this for the rest of the day
Maybe you can find anything in the links. Good luck