Is it possible to add a backdoor number to a Partner mail VS R5? If so, how? thanks.

Just add another line to your system. Asign it to the auto attendant. Give all the phones NO ring. (Actually, give the Operator DELAYED ring, so if the VM can't answer, the Operator eventually gets the call). If you have an ACS, make the VMS Hunt Delay for the line be 0.

Now your internal people call the "backdoor" line, get the auto attendant, and can dial an extension directly or press *7 to log in and check messages.