Friendica doesn't use the "!" notation. This is only a notation that a Lemmy, Piefed, or MBIN instance understands.
Friendica instead uses the familiar account notation used by Mastodon, Sharkey, Misskey, etc. It sees the Lemmy community as a user that is also a group.
If you want to follow, for example, the "asklemmy" community at lemmy.world, in the "Add New Contact" box, just enter : asklemmy@lemmy.world, and then click the "Connect" button, and it will find it, and you can follow it like any other account. Friendica will recognize it as a "Group", as opposed to an individual account, and it should show up in your Groups section.
Let me know if this works.