You don't need a system to be in the dropdown list to have a character sheet for it; ask in the Sheet Request Thread, Consolidated, mk I
thread for someone to help you make or make a character sheet that can
then be added to the list of character sheets available to users for
their games. You don't need a system to be in the dropdown
list to play it; use 'Other Games' and put the name of the system in the
listing text, all the forms of it someone might use. Actually play the
game. Ask those you know who play the game to do likewise. You don't need
a system to be in the dropdown list to ask the writers of a system to
think about licensing their system for use with the Roll20 VTT; talk to
whoever produces the system you're interested in, tell them how
interested you would be in being able to have an Official Character
Sheet and Official Compendium available on Roll20. Ask those you know
who play the game to do that same.