I need a 1-800 pointed to an Asterisk server and then forwarded to a standalone SIP IVR that we have to use as it is a legacy system that we can not replicate using Asterisk IVR. How would I go about doing this?
Yes you can do that.
Define a peer on your asterisk box and dial to the legacy machine.
you can define insecure=yes (if your machine is not open for public) and route the calls with DID number to the legacy machine.
insecure=yes is an outdated way of solving a problem that is not implied by the question.