Hi,
Definitely you can raise individual PO for each task & assign these to the maintenance order. For planned jobs, you can assign a specific operation as 'External Operation' & assign the vendor to the operation along with the service rate. When the maintenance order is released, the PR will be created & the PO can be created with reference to the PR. For unplanned jobs, create the task list manually & assign the vendor to the operation marking the operation as external operation.
This can be a tedious work, as for each job, a separate PR will be generated. Alternatively, you create a manual PR with assignment of cost center of maintenance department & the cost can be loaded to the maintenance order via overheads. Please consult your CO person for this.
Hope this is clear & resolves the issue.
Regards,
Prashant