Hi Mark,
A correction - there is an OOTB "Import Requisition" task available which can be used to import requisitions from external system like ERP or other system. If you are looking for such kind of requirements then you can use this task to accomplish the same.
Though if I understand correctly you want to populate line items while creating the requisition, thus you can customize that accordingly to populate the line items.
Thanks
Sandeep Jain