Hi,
Have exactly the same question.
Did you find a solution? I'm wanting to add custom symbols to my F6, but I haven't figured it out yet.
As far as I can tell, custom is not a feature of this device. It's hard enough to use the built-in ones that are available (first aid, left, right, straight, summit...) that if I need them I have to use the old Garmin Training Center to make the file. Maybe plotaroute did them but it was hard to figure out.