Limit the list of users on Portal while substitution

Hi Experts,

We are using Portal 7.4, and want to apply substitution rule. Currently end user can select any of the user as substituted one, is there's a way to limit the list of users you can select from when you are creating your subsititute rule?

For example, Manager should only be able to create a substitue that is at their level and restrict users that are below them and should not be  processing tasks.

Please help.

Thanks and Regards,

Khushboo Agarwal