"I'm trying to find out where I can create custom fields so my uploads can match and associate Order IDs to the trucks carrying them.
Please let me know if this is supported. Thank you."

When exporting reports to Excel to view or edit, you can customize the data that is included in the Excel report but you cannot merge or add external data to the Excel doc and upload it back into MyGeotab.
Contact your account manager to discuss custom data solutions or a possible marketplace partner service.