Hi Dominic,
Just drag the spreadsheet on the Data tab in Service Studio. OutSystems will automatically generate an entity from the spreadsheet plus logic to load the spreadsheet rows as records into the entity when you press the 1 click publish button.
See https://success.outsystems.com/Documentation/11/Developing_an_Application/Use_Data/Bootstrap_an_Entity_Using_an_Excel_File
Regards,
Daniel