Is it possible to add a day to our 6 day base tickets if you've already linked it to MDE? Thanks!
Yes, it sure it.
Linking your ticket doesn't change your options, only USING part of a ticket changes the options.
If you have not yet used any part of a WDW ticket, you can modify it at any time. An unused ticket essentially gets traded in for another ticket of the length you need, and you can also add the hopper or water park options. You can even trade in a ticket with options for a new ticket that doesn't have those options, as long as A) the ticket you're trading in is completely unused, and B) the ticket you're trading it for costs MORE than the ticket you're trading in. You can't trade a 3-day hopper for a 3-day base ticket, but you could trade a 3-day hopper for a 6-day base ticket (they're exactly the same price) or for a 7-day or 8-day base ticket.
Once you've used any part of your ticket, your options then become more limited. You can only add days or options to a partially-used ticket, up to the ten-day maximum ticket length. If you add the hopper option, it's a flat fee - they don't knock any off for the non-hopping days you've already used. Even if you only have one day left of your base ticket, adding the hopper option still costs $55.
You can add days or options to a partially-used ticket at any time until the ticket is completely used, or 14 days after the first time it was used, whichever comes first.
You can upgrade any ticket to an Annual Pass. Again, a completely unused ticket could be traded in for either the Platinum or Platinum Plus AP, but a partially-used ticket with the water parks option can only be upgraded to a Platinum Plus AP, even if you haven't used any of the water park admissions.
And it goes without saying, any change to an existing pass must be done at WDW, at Guest Relations or a Ticket Sales window at the theme parks or DTD.