Hi,
You can add members to a queue, as the queue is a native asterisk application with a proper api to work with.
Ring groups are not made with an application, but with custom dialplan recipes.. as such, there is no native asterisk api to work with them.
FOP2 is an Asterisk application, (not a FreePBX application), for that reason there is no way to modify the FreePBX configuration from FOP2 itself. You have to use FreePBX for that.
In any case, you can very well replace your ring groups with queues, they behave exactly like the same.
Best regards,