There are utilities around like "touch" that allow you to modify more or less any of the dates of a file and I am sure that you can this utility as a tool from within MP3tag.
Please note that the OS is in charge of the file properties.
MP3tag's design is open enough to cater for your special need: create a tool entry.
There has even been a German thread that deals with it. Look out for DetlevD's vbs script. You will recognize the tag field where to put the desired date.
File-Datum zuweisen / aus Tag generieren