Can a Mii be transferred to the Switch from another Nintendo console?

Yes, but only if you have an Amiibo to transfer it (which it sounds like you already figured out).

Nintendo answered some questions Kotaku had. This was in one of the responses.

You can send/receive Mii characters through local wireless communication between Nintendo Switch systems, or use an amiibo figure to transfer Mii Characters from Wii U and Nintendo 3DS systems.

Having said that, it sounds like this is a uni-directional transfer as Switch Miis have some new properties that previous Miis did not have.