Mole machines can already tunnel to Outland and Draenor, so there is no reason they couldn't tunnel to Shadowlands. Draenor is not only across space like Outland, but also across dimensions. Shadowlands is also just a different dimension.
However, I don't think they will add mole machines to the current content. Like in BFA, we saw that it was one expansion behind. With that in mind, Zandalar and Kul Tiras mole machine locations should be opened up.