Do you know how to set up a portal to a related table?
If not, that will be the first thing that you need to learn. Portals are extremely common design elements in FileMaker Pro.
Many thanks - yes since posting the query I'd just figured how you can edit data via portals (I originally thought these were just for displaying data), so I'm on my way with this now :)
You can edit data in a portal and also create new related portal records as well.
Do you need any further assistance? A looping script can do what you want, but often, the ability to add new records directly in the portal is all that you really need.