[Freeswitch-users] joining a bridged call to a conference

hello, i got an established call A calling B. A is a number outside FS (any number e.g. 16084191500), B is an extension registered to FS (e.g. 1002). after some time, i would like to join the call A => B to a conference roon e.g. 3001 (of course using ESL) any idea how to do it smooth ? whats the best practice for this ? i tried uuid_transfer but whatever i do it keeps killing one leg... thanks for help, T.