If you just change the subreport expression to "repo:{report_name}.jrxml" then when you upload the parent tag in the jasper server webpage (or from jasper reports/studio ide) you should be prompted to upload the subreport file.
This is covered in the Jasper Server user guide in the docs folder of your install e.g.
The server detects that the report unit includes a subreport, and the subreport needs a different
expression. In iReport, subreports need this expression:
$P{SUBREPORT_DIR} + reportName.jasper
In the server, subreports need this expression:
repo : reportName expression
c. If the Subreport resource is not selected, click the checkbox to select it, then click continue to
upload the subreport and the proposed expression to the repository.