On each call handler, under the Caller Input page, there is a checkbox "Allow callers to dial an extension during greeting" that will let your users punch in an extension to be transferred. The catch-all, though, is that Unity must have the extension we're trying to transfer to in its database first. We get it in there \by assigning it to either a call handler, internet subscriber, or regular exchange subscriber.
Now, just in case I misread your original question and you instead mean to have the opening greeting instead drop someone off at a different extension when they're supposed to get the opening greeting, then you need to make sure that everytime someone moves to the opening greeting from any other call handler or subscriber conversation, that you're doing an Attempt to Transfer to the opening greeting. You can configure this on the Caller Input page where you set a key to "Send caller to Attempt to transfer for Opening Greeting."
If you want someone to get transferred immediately instead of hitting the opening greeting when *dialing* into Unity, then you'd need to select "Yes, ring a subscriber at this extension" under the Transfer Incoming Calls part of the Call Transfer page on the opening greeting. The call routing rules that point to the opening greeting are already configured to do Attempt to Transfer.
adam
