Hey there,
It’s a pretty tricky scenario. I have a list of Users that come to the list from relation.
The case: Once I click on this user, I want to show a form of adding a record to another table that should use user RowID plus some additional data.
But it looks like this List can manage only the context of entity that it dsplays.
I’m not sure I completely understand. Do you have some screenshots showing what you are trying to do?
My initial thought is that you may need to add some value components to your form, but I may be misunderstanding the problem.
Would a “show form screen” action on each User in the list do?
It was my initial idea, but it doesn’t allow to work with entities apart from “User” 
Interesting, if the data source is from the table directly, then glide allows us to do that.
That’s weird. If you’re showing that relation in a collection then that should work. Can you show us how you’re configuring things?