You can only do that within an analogue phone, that has the option (then connected to an ATA). SIP numbers are sent as a complete "package". The 3CX phone cannot insert a pause after sending a SIP message then change to send DTMF, yet.
http://3cx.ideascale.com/a/dtd/Add-a-pause-in-a-dialed-number-in-3CX-Phone-client/309646-9854