I want to disable to whole Game UI to create my own UI.

The question is, would I need to use triggers to detect if a player press his Ability button or is it possible to make the Dialog act exactly like the Command Card (except that its moved somewhere else and doesn't look the same) ?

You would add a dialog box that acts like the command card.

I suppose you know how to remove the current UI, and add dialogs etc.?

Yup.
How is my question.

I remember seeing something where you can create a dialog template using the command card template.

http://www.sc2mapster.com/assets/dialog-item-templates/ You may want to look at the cons to using this kind of approach

The fact that disable Game UI disables hotkeys is kind of a major issue.
I guess I'll just use triggers to detect key-press... or would it be too laggy? :S

Question is, how fast do you want the response. The faster you want to detect, the more lag it will create.

EDIT:
Also, from the link Temp posted:

Post has been edited 1 time(s), last time on Sep 12 2010, 2:18 am by NudeRaider. Reason: merged posts

I read this, and thought it made sense... but then I understood that I did not simply want to move the Command Card. I want to have it have a different form also (5 buttons aligned on a single line).

I believe it is also possible to set the game UI to a really really small opacity, practically making it invisible but still function. Then you could layer dialogs on top of it.

As far as lag with trigger key press detection, that small amount of lag will always be there (somewhere between 50-150ms).

I have already tried to "Set Transparency to 0" for a dialog box, nothing happened. I wonder why it'd work on Game UI. ;o