Here is how. It seems longwinded, but is fast when you get the hang of it. It's harder to write than to do!
First Create two new tracks, named EDIT and MAKE FILL (what they are named is irrelevant of course).
#1 Ctrl+alt drag dialog so you have the scene you need to create fill from on both EDIT and MAKE FILL tracks.
#2 Run detect silence on the Scene on the EDIT track.
Make sure settings are set low enough that not just dialogue is kept, but also large movments, clothes rustles and footsteps as well. Also make sure the pre/post roll is set at the minimum length you think might be needed to create good sounding crossfades. Click process.
Result: only the unwanted material is left.
#3 Check preferences, delete overlaps pref ON.
#4 Ctrl drag the selected unwanted material over to the MAKE FILL track (thus staying in sync).
Now all the unwanted material is SELECTED and all the fill parts are left between the selected events.
#5 Now delete all the selected unwanted material (just hit backspace).
Result only fill material is left but with a lot of holes in it.
#6 Enable snap+shuffle.
#7 Select all the fill events, but not the first event. drag the selected event to the end of the first event and you will now have your continous selection of fill material.
Now I would quickly listen through the transitions (with a slight preroll set in my transport panel), deleting events/trimming away parts that doesnt work well as fill.
Crossfade to taste. Dont apply crossfades longer than your set pre /post roll in detect silence or unwanted sound will start to creep back in during the crossfades.
Large parts of the above can be handled using macro and PLE making it a pretty easy and fast process.
Yes the explanation is longwinded and seems complex, but it isnt hard or complex when you've tried it a few times. The hardest part is actually setting the detect silence thresholds properly.
Here is a gif showing how to: