Zandronum

Is there a way to add an entry to an existing menu without totally replacing it ? (Like the block "addoptionmenu" that works with gzdoom) If not, what is the best way to do such a thing ?

Not possible in Zandronum. You need to resolve to replacing the whole menu until Zandronum gets updated.

The new version of survivalist has a new option in the menu, i don't know if its replacing anything though.

It does. Zandronum has not/barely been updated since my last post, so it's still not possible to append a new menu item.