As far as I am aware, MS has never said you can't import aircraft. In fact, I've read of developers considering just that. Unless you want an "Easy Button" to automatically convert an aircraft on your own - that does not exist. But that's not really how this works. You're talking about "importing" content built on a system ~15 years old. FS2020 isn't just a slight every other year update, it's a whole new ballgame. To convert a plane properly will require a good bit of work on the model, textures, and flight dynamics. The small caveat is there is a "legacy" mode in FS2020 which would presumably use the old flight model, but even then there is still a lot of work to be done visually.