Partner Mail VS R4 does not have submenus, but it does have Announcements.
To transfer from an announcement to an extension, you have to dial *8 (*T-ransfer) first. So make selector code 5 go to Announcement 1, and have announcement 1 say "For the Shipping Department, dial *8-11. For Accounts Receivable, dial *8-12....."
Make selector code 6 go to Announcement 2, and have it say "For Fred Flintstone, dial *8-10. For Barney Rubble, dial *8-11....."
For Selector Code No. 4, you want it to transfer to a specific extension? Use Selector Code Transfer, and specify the extension you want it to go to.
Selector Codes 1 through 3 keep as Direct Extension Transfer, so you can dial extensions from 10 through 39 directly from the auto attendant.
DND - calls transferred from the Auto Attendant to an extension in DND transfer to Ext. 10 instead of the mailbox? I think you don't really have an R3 ACS. Since it is an R4 VS Mail, your only solution is to NOT use DND!