I don't think there is a way yet, not without starting your own "private" magazine. But it would make sense to have a section in your user profile where you enter a list of tags you want to follow, just as there is a way of adding a list of users that you follow.
Although... it occurs to me... I'm not sure how kbin actually uses that list of users you follow. But wherever/however that is, it would make sense to add followed hashtags also.
I think Mastodon support in general is still very rudimentary at the moment. Would be great to see more interoperability between the two (from both sides).
Followup: I created a Scuba magazine, and added a few hashtags to it. It pulled in a few posts from mastodon, but not as much as I'd expect, since my mastodon account that follows the same hashtags sees a lot more action.
I presume my magazine will only grab toots from federated Mastodon servers, but since I'm not following them directly, I'm not really sure how to force federation with them, or how to tell a bunch of Mastodon servers to send their toots our way.