No, you don't plug in a NAS to a router, you simply plug in any old USB drive to the router to "make it into a NAS."
Yes is doable but has its limits. Lots of time the transfer rate is less than what you would get out of an ethernet "honest-to-God" NAS. Read the user's reviews as to exact number as very often this is omitted in the vendor's spec.
Then it maybe picky what disc format, NTFS/FAT etc it wants.
Some have Read Only capability.
But just to get your feet wet, sure.