How to automate import employee expense reimbursements?

Summary:

I'm not sure if i can create a Payroll Flow and reuse another existing custom Payroll Flow or do i start from scratch. Either was I don't know much of Payroll Flows and whether this is the answer to the need. I am looking to automate importing employee expense reimbursements from and external system. The steps would be the external system would generate a weekly file of employee reimbursements and place the file on our external/internal sftp site. HCM would at a schedule date time, would obtain the pipe delimited flat file from the sftp site.