You would set your game to be 'Other', then be certain to mention the system in the description of the game. Searches for the game name would then return your game. You could also advertise on social media for the games you are running, and ask fellow GMs of the game to be sure they are advertising their game and playing it on Roll20. Finally, you could encourage the publisher of Fragged Empire to look into partnering with Roll20 in some way, possibly by releasing modules on it (as Wizards of the Coast and Pelgrane Press have) or similar.