Yes you can use User master parameters to default the a value in Requisitioner field. You will need to add a custom parameter to the table TPARA using FM RS_PARAMETER_ADD. Maintain the parameter ID so formed in SU3.
Then, modify the data element of AFNAM in SE11 (this will require an access key).
For your second question, the workflow change should not be a major change at all. The workflow just needs to look at a different user (and then HR) record now to determine the manager.