Used cars, trim levels, add ons....?

Sometimes they will say "yes" but the result will be less than OEM. Easiest example, sunroofs.

OEM factory unit slides under the sheet metal and has six drain hoses.

Dealer-added slides over the roof, creating a LOT more wind noise, fewer drain hoses (if any), and is more prone to leaking.

Some things are a relatively easy swap, such as the leather-wrapped steering wheel.

Some things are possible but require a little more work, such as swapping in heated seats.

Some are, for all intents and purposes, impossible, such as upgrading to factory navigation or autoheadlamps.. Too many sensors to make it feasible

I seriously doubt that you'll find any dealer willing to do anything like this. Even on brand new cars, dealers are limited as to what they can add to a car -- certainly not major features and options that upgrade from one trim level to another.

Yes, you can always add leather, power seats, Naw, etc... But It's cheaper to find an sel. I would suggest saving my money for a little while and buy what I want.