I have an SV8100 system .. My question is:
If an outside caller calls in and makes it to the extension's voice mail, can the caller exit and dial another extension while in the voice mail?

Basically, I want to leave a greeting the prompts them to dial another extension or leave a message.

Yes, press # then that should kick you back to the main AA where you should be able to dial another extension if your DAT is set correctly.

Got it thanks!

Or enable dialing option and assign a NCRMB that has a DAT that can transfer.