Hi Gavrav,
Sorry for late reply. To capture machine activity only, on the work centers where you are using subcontractors, define "Activity Type" for machine in "Costing" tab and leave it blank for labor. This way, you can capture operation time for both machine and labor, but only capture cost for machine.
I am not sure about charging using subcontracting PO, you better discuss with your purchasing and CO team.
Regards,
Julian