You could use Replace Field contents to insert data into the same field of all the records in your found set. That might work.
You could also re-import your data but select the Auto-Enter option on so that the current date is inserted in the date created field if it's set to auto-enter creation dates like I suspect it will.
With both of those approaches, it's a good idea to make a copy of your file first in case you get results that you didn't want.
or you could possibly just type in a date in the field just before you try to add a note? (the layout may or may not permit this, I don't have a copy of this database file at hand to check.)