This question seems to come up every few months.
The single player (SP) games are saved on your 'puter running FL. Multiplayer (MP) saved games are saved on the FL server. While you as the client have some of the files to run FL (mostly visual and operating details, not accumulated details like cash, ship type, equipment, etc.), it's the server which brings it all together between players and their characters. Thus a player cannot reach his MP saved game file but as long as the FL server is in operation and you play regularly (on average every month or three) then that character will be there next time you log on.
Few sysadmins, if any, will accept your SP saved game file and insert it in their FL server. While the files are interchangable it takes some setup and a whole lot of trust (trust your file doesn't contain say equipment you aren't allowed to have on the FL server) to make it work.
Besides from the majority of FL MP people, FL is easy enough to level up.
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Earendil
SysAdmin of Boston Freelancer server
Server community website & forum:
EarendilsPlace.net (
server rules)