For this I would create a Sublist Action Group set to Items, then create a Return User Error action using the criteria of Item = specific service item.  It would throw a warning with whatever text you want – something like “You must populate [enter field name here] before saving.”  It is not technically the same as the set mandatory field action, but should achieve the same result of restricting save when not populated.  Of course you would want it to trigger as a Before Record Submit to actually prevent the save… hope this helps!

You can also import via csv in a single file – no separate header / line item file required.  Depending on the method and format of the data you are receiving to create the orders, you can develop an API or use an IPaaS of sorts to import via sFTP or other protocol.  If there is a set standard for the orders, as in always the same line items, you could potentially use a memorized transaction as well.  Either way, the good news is there’s options.