Unfortunately, no. The purpose of a game mode is to provide a consistent, reliable user experience that is what the author of the game mode intended. This means loading a known set of add-ons in a known order. If you want to load a custom set of add-ons you should start a custom game or make a new game mode.
The problems I'm trying to address here are that users are constantly having problems with conflicting add-ons and content creators are constantly getting false bug reports that are the result of add-on conflicts. The gamemode system is an improvement for both parties.