Adding the portal won't destroy the existing data, but you'll find that your existing data is now in the wrong table. A portal opens up a kind of "window" into another table's records via a relationship you define. Once you've defined your related table, linked it to your original table in a rlelationship and then got the portal in place on your layouts and working like you expect, you can use Import Records to copy the notes data from your first table into the portal's table. Since there must be a relationship linking the two tables, you'll need to import both from the note text and from whatever field or fields in your original table define your portal's relationship into the matching key fields of your portal's table.
So if your relationship in Manage | Database | Relationships looks like this:
Main::ID = Notes::ID
You would import both from Main::Note to Notes::Note and also from Main::ID into Notes::ID so that the new records created by the import match up to the correct records in Main.
Warning: Import records has a lot of options to choose from when you use it. Look it up in FileMaker help, make a back up copy of your file before you try it out and don't hesitate to ask questions if you can't figure this out.