If you want to modify a dial-plan, you add something like this:
SipAddHeader(P-Asserted-Identity: sip:${CALLERIDNUM})
If you want to do it in a dial-hook, you could use this:
If this is a one-off, you could create a new outbound route and specify the outbound caller ID in that, and set it so that only your extension is identified as a possible caller.