For PolicyXpress actions, for type of User you can only update the subject of the originating task and you cannot specify another user in the same way that you can do so with Data Elements so if you want to generate a modify against another user via PX Policies you would need to look into the Rest/Soap/SQL/ExternalJava approaches. A KB regarding External Java code to review would be:
Explaining PX Policies (Invoking Of External Code) - CA Knowledge
Alternatively you could look into the Identity Manager APIs (i.e. Event Listeners) to code up a solution instead of trying to use PX Policies.
You can open an Idea within the CA Communities as well concerning the ability for a PX Policy action to specify a user other than the subject of the task.