Use of CATS for External Contractors - costing

Hi I am currently implementing CATS as part of a large implementation. Time writing for internal employees has no issues. But the client would like the external contractors rates to be picked up from the service/purchase orders, rather than maintain multiple activity types rates. We are not implementing full HCM in this phase of the project

Is this possible?

If yes, can anyone please advise how?

many thanks

Siobhan