Unfortunately, what you describe is impossible with our products :-)
The Transfer to Voicemail actually sends the call into the voicemail system, where it is recorded and put in a mailbox. From there the system would have to be monitoring the mailbox to pull it into ACD. Interaction Attendant's Transfer to Voicemail operation is unable to send the voicemail directly into ACD.
So, you'll need to explore a bit further to find out what is monitoring the mailbox associated with the workgroup (I am guessing that Routing is configured on the ACD tab of the Workgroup configuration for that workgroup), then see if it is a Custom type or ACD type workgroup (which determines where to modify the behavior).
Or, it's possible there is an EMail Attendant profile set up to monitor the mailbox.