After you create the attribute, you'd have to add a constraint to limit what it can be set to. To learn about constraints see: https://support.ptc.com/help/wnc/r12.1.1.0/en/index.html#page/Windchill_Help_Center/typeattr/TypeMgrAttrConstraintRuleRef.html
We've done something similar to this using Enumerated Value List, but we do have to update the list to add/remove users as needed.
It might be interesting to try using the "Legal Type List" with the type set to User ("wt.org.WTUser"). I haven't tried it, but I wonder if it would work.
