I was trying to do the same thing--have original dates for notes that I was moving off my old Samsung phone onto my Mac (which syncs with the iPhone I now use). Both programs display one date--either the created or last modified date.
The solution I used: I sent all Samsung memos to my computer (I actually put them in Notes.app, but it doesn't matter where you have them on the computer). Then, what I did is I turned off the Wifi (maybe not necessary?) and went into "Settings" > "Date & Time", unlocked the lock at the bottom and unchecked "Set date and time automatically". Then, I reset the date to the date I wanted displayed on the note in Notes.app, clicked "Save" and I made a new note, copying what I had already imported into the computer/Notes.app. It created the note with exactly the date I wanted. :) I repeated the process for the rest of the notes. At the end, I rechecked "Set date and time automatically" and restarted the Wifi.
A solution that works, both clunky and elegant, depending on how you look at it. I am going to do the same for importing my voice memos into iTunes to sync correctly with Voice Memos on my iPhone.