Not join but add…
Select a fill.
Switch to the tool "fill"
Begin to edit your fill.
Select the "Add a fill" —with a +…— in your contextual menu
Space bar clic on the contour of the fill that you want to add at your first one.
The first fill is increased by the second one
Don't forget to remove the second fill —which don't disappear in the operation—.
The two fill can have differents attributes…
You can do that with a fill and different elements : slab, roof, polyline, etc. —everything which can space bar clicking…—
You must recreate the hole(s) —if there is…—of the second fill to keep them in the result —by space bar clicking…—
Hope this helps
