Using AME for purchase requisition approval

We want to start using AME for purchase requistion approvals.

We have an approval policy that based on a hierarchy, so it fits well with AME.

However, for purchase requistions tied to projects, we would also like to capture the project manager's approval

The project manager may or may not be part of the existing approval hiearchy. 

How can we make it work that the requistion is approved by both the project manager and the members of the hierarchy?