As far as I know the only way Honda ever lets you choose a different wheel/tire than standard is as a dealer-installed accessory. That's the best case here, an official option in the build tool that lets you choose an 18" wheel as an accessory, but you'll be paying extra for those and then it's up to you to sell your 20" wheels or try to get the dealer to give you credit for them.
I think since most non-offroad people think of 20" wheels as an upgrade it won't be too hard to get a dealer to swap wheels as an even trade. I don't really see how this is any worse than being able to order it from Honda this way since basically every option on a modern Honda (other than trim and color) is dealer-installed anyway.