No, but there are alternatives that produce what is effectively the same result.
Specify a sort order for your portal that puts a new portal record with all empty fields first in the portal. Then put a button on your layout that performs a script to create a new related record in the portal. When you click that button, a new blank row will appear at the top of your portal.
With this option, you can remove the "allow creation..." option in the relationship and this method can still be set up to work.