We ran into an issue similar with needing some additional functionality out of the Exchange components outside of what was provided. We ended up using the Exchange Management Shell and using Workflow to dynamically create the powershell scripts to create and run everything. That's been months ago and since then I've gotten a little more familiar with Web Services which is another option. I haven't looked at Exchange's SDK but that would be a direction to go in.
For this particular one you could use the "set-mailbox" cmdlet to do the work for you. So from a strictly command line perspective your command would be..
set-mailbox old.user -ForwardingAddress new.user
From the workflow perspective you would want to use a create text file component which would create your .ps1 file (equivalent of the batch file basically) and it would be populated with the necessary variables for your users.
Then a write file component to write it to a temp location.
and lastly an ExecuteProcess to call the powershell executable which calls the temp ps1 file you created earlier. Which would be something like..
C:\WINDOWS\system32\WindowsPowerShell\v1.0\powershell.exe -psc "C:\Program Files\microsoft\exchange server\bin\exshell.psc1" -noe -file "c:\path\to\temp.ps1"