Here’s how you can quickly make an LFE channel:

copy the main tracks which contain some bass, for example the music tracks (or even all the tracks together if needed) into a new mono track
run a lowpass filter at 60hz
mix the volume level so it is at a reasonable volume and comparable to the LFE track in other scenes around it
this is your LFE track, it’s literally just rumbling bass!

The reason I asked is because I heard about this tool which is supposed to work wonders for converting from 2.0 to 5.1: https://nugenaudio.com/haloupmix/
and was wondering if anyone here had tried it or something similar.

Sounds like you are a wizard if you did this manually in Audition!