The problem

There is no way (afaik) to exclude a folder by using a file dialog

Requested Solution

Sources View -> Manage excluded items -> Custom Tab -> Click "+" Button.
The very similar little Files/Folders/Paste Context Menu pops up for just excluding stuff.
This little context menu should of course preserve what the "+" Button does right now.

Or add the new exclude file dialog button globally in the "patterns to exclude" view window.
There should be plenty of space in the bottom left corner. Although you'd either have 2 confusing "+ Buttons" or change the current functionality of the "+" Button

Alternatives

Copy and pasting multiple Paths like a caveman gets old really quick.

Additional context