It has no official support for adding new fields. You can try forcing anything you like, but it's going to be work. There's triggers for event edit and event display, but none really designed for adding new inputs to event edit. There would also be no storage for those custom fields so you'd have to deal with that issue as well.

Best way to add new inputs is probably through the template files then handle storage on gj_onAfterCreateEvent and gj_onAfterUpdateEvent to store them into the params column. As for displaying the custom fields you'd act on gj_onDisplayEvent.

Both the GJ Activity and GJ Gallery system actions add new tabs to groups. Recommend reviewing their output parameters to see how to add a new tab to groups.

on the same subject, can you tell me what's wrong in this substitution ?

Did you enable substitutions for the output? The parameter should be visible under the Layout parameter. By default output doesn't send through substitutions.

Also how can I get something like :

You can't. Nested substitutions are not permitted. You can nest substitutions inside of format functions, but not substitutions inside of substitutions. We're actually working on implementing support for this in a future release though. Feature ticket can be found below.

forge.joomlapolis.com/issues/7828