> Is there an option to position this "add row" at the very top, instead of at the bottom?
Short answer: no.
But one of these alternatives might help:
(1) create a script with a Go to Portal Row [Last] step, so that you don't have to scroll through the portal to get to the last row.
(2) create a global field and put it above the portal. This global field will be a data entry field where you can type the data for a new portal record. Then create a script that sets the contents of this global field into the relevant field in the last portal row.
Here's a third option. Use this script:
Set Variable [$ID ; ParentTable::PrimaryKeyField]
Go To Layout [PortalTableLayout]
Set Field [PortalTable::ForeignKeyField ; $ID]
Go To Layout [Original Layout]
If you specify a sort order on your portal, that sorts such a new blank record to the top of the portal, clicking a button to run this script will automatically add a blank row to the top of your portal.