Unfortunately, Itch payments don't work that way.
If someone had bought the main game for $15, then just paying an extra $5 won't add that $5 on to the first $15 to allow them access to the main game + DLC for $20.
That person still needs to pay a separate $20 to get the game + DLC.
I also bought the main game here, and just paying the difference in the price of the main game and of the game + DLC doesn't work.
Maybe you could sell the DLC separately on Itch for those who already bought the main game here?
But, I guess that depends on how easy it is to manually install the DLC onto an existing main game installation.
Thanks, and I'm looking forward to your continuing good work on your games!