Hello, How are you trying to do that exactly? If the following doesn't work, then maybe it's a limitation of non-GOG games for whatever reason.

1. Click the "More" button next to Play/Install, then expand Manage Installation, then choose Configure.
2. Click the Features tab.
3. Check "Custom executables / arguments".
4. Click "Add another executable / arguments" and select the executable in the file browser window that pops up.
5. Make sure "Default executable" is selected for the new entry, then click the OK button to finish.

Thanks for the reply.
The custom executable is grayed out on any epic store game for me. I was able to add the bioshock exe by first de-syncing from epic, which afterwards returned the page to it's default state (as if I never owned it). Then after re-syncing it's now listing both options. I just hope epic fixes their damn store. The only good thing about it is the free games.