What you’re looking for is mbin.

Mbin supports both lemmy and mastodon.

Make an account on https://fedia.io/ or something

Lemmy only supports following groups. It will reject a post if it isn’t posted to a group.

Check out piefed and mbin for things that are more compatible.

From what I can observe lurking, communication between Lemmy and Mastodon is pretty half-baked, with things overall boiling down to users on Mastodon being able to follow Lemmy users, but not the opposite, and posts from Mastodon users may appear on Lemmy but with the text body appearing as title instead. Issues like these may appear due to how each site engine handles the data received. If you must follow users from Mastodon on Reddit-like forums, Mbin is far more compatible.

You cannot follow everything from everything.

• From Mastodon you can follow Lemmy communities and Lemmy users. Some Lemmy communities name gets hashtags on mastodon, e.g. if you follow #idm there, you will see posts from !idm@lemmy.zip
• From Lemmy you cannot follow/subscribe to anything from Mastodon. You can see replies and posts of Mastodon users in Lemmy communities

If you want to follow all from one account with a Lemmy-like interface, the closest you can get is with Mbin. There you can follow Lemmy communities, Mastodon and Lemmy users. You cannot follow Mastodon hashtags though.