In Lutris there's a "Command prefix" configuration option both per-game and one in the global config with the default for all games, which is where the firejail command line goes (basically for sandboxing with firejail you're supposed to run "firejail firejail-options original-command original-options" and putting firejail and its options in "command prefix" does that).

Note that there are other sandboxing options that run in the same way as firejail but I found firejail to have more straightforward options.

Also note that this won't sandbox the actual setup of a game, only the running of the game.