Here is a link to the DNIS ANI substitution for your product:Enhanced ANI and DNIS Substitution in AOS Voice Products if that does not answer your question please provide a call flow diagram showing what you are trying to do, it maybe possible to get the needed results using some other configurations.
There isn't a way to automatically send an extension. If the extension maps to a dialable DID, you can substitute the entire DID.
Some PBXs and autodialers have the ability to dial a number, pause for an autoattendant, and then outpulse DTMF to ring an extension. Is this the functionality you're looking for?