Restrict the number of users to whom workflow notification can be reassigned.

Hi,

Service Request notification is coming to SR owner after creation of SR. Owner can check it in worklist and reassign it to some other employee.
We are having a requirement that a particular user can reassign the SR notification to his/her supervisor only.
Can this form be personlized for restricting number of users?
Can we restrict the number of users to whom notification can be reassigned (i.e. can we control the LOV of users for reassignment).

Thanks & Regards,

Gaurav Joshi.