You can follow them from your already existing Mastodon (and maybe kbin?) account.

From my account on mastodon.online I just followed https://social.overheid.nl/@beheerder as a test, and I've already been following https://social.network.europa.eu/@EU_Commission

For some reason my server couldn't find users from the social.bund.de when I pasted the follow-link (like https://social.bund.de/@Zoll )

By the way Mastodon has a very nice interface to subscribe to other instances. Like now when using when following the link in OPs post and opening a web browser, then clicking on a user and clicking follow, it gives the option to sign in to subscribe OR copy a link to subscribe from another instance . Then I just paste that link in the search field in my Mastodon app (logged in to mastodon.online). Hopefully Lemmy will implement that "button to copy link to subscribe from other instance" soon