Special User Account Question

[Sean Murphy]

I would like to setup a FreeBSD user account that has specific rights to 
certain files.  This "special user" should not be able to login or ssh 
in.  Certain other user accounts must first login as them and then use 
"su" into the special account (to become that special user) thus gaining 
access to specific files.  This is a situation where a group or group 
permissions will not help this problem.

Any ideas?

Thanks