RSS fans!
You can follow any Mastodon account via RSS by just adding ".rss" to the end of the account's public profile URL.
To find a user's public profile URL, click on their profile image within Mastodon.
For example, if you click on FediTips' profile image you get this page in a new tab:
http...
I use the FediBuzz Relay to push some hashtag-specific content to my instance. When you set that up, FediBuzz creates a "user" that boosts anything with that hashtag. For example, one of the hashtags I want it to help look for is #HoosierMast. It created @tag-hoosiermast@relay.fedi.buzz. So, maybe that is a work-around? However, I have no idea how to then take that Mastodon username and add it to your follows on Lemmy!