Something like this will get you started. It may have unintended consequences:
globals_custom.conf:
TRANSFER_CONTEXT=xfer-prepend
extensions_custom.conf:
[xfer-prepend]
exten => _.,1,Noop(Entering user defined context xfer-prepend in extesions_custom.conf)
exten => _.,n,Set(CALLERID(name)=prefix:${CALLERID(name)})
exten => _.,n,Goto(from-internal-xfer,${EXTEN},1)