Boosteroid allows you to use only original video games, hence, such modifications as FiveM, Roleplay are not supported. Installing mods can be difficult or impossible, up to the closure of the game session (Boosteroid monitors unauthorized user actions and closes the game session without warning).
Steam Workshop, when starting a video game on Boosteroid, creates a UserId folder in the Steam folder of the base Boosteroid image.
Given that space on the disc is limited and depends on what new virtual machine you get connected to, a user with a large number of modifications will either not be able to launch a video game, or it will take a long time to download such modifications.
Keep in mind that each new game session on Boosteroid is a new reference virtual machine, and the installed modifications will be removed when the session gets closed.
Generally, it depends on a video game and its developers whether Steam workshop would allow you to download mods.