Yes, using the SM module in WHMCS we had to create the site manually in IIS so I can confirm that too.
To use WSP with WHMCS for SM:
Install WSP on the same Windows server
Add the SM configuration (admin user/password/IP address etc.) under servers
Add the WSP module into WHMCS
Configure the WSP module with your WSP server IP address etc.
Create email Hosting Plans (like products) within WSP
Create products using the WSP module within WHMCS
The above will allow full mailbox creation and administration, including suspensions etc. all from within WHMCS.
When setup it works well, however, it is another link in the chain that really shouldn't be needed IMO.