The old black powder shotgun I used to have used felt like pads between the powder and shot column. Some of the pads that were available were lubed with a non sticky wax, very similar and maybe even beeswax. Then of course, there was another pad or sometimes, if the pads were hard to find, wadded up newspaper to hold the shot column together.
I tried using double AA type plastic wads in the shotgun with felt pads or news paper over the shot. It worked well for a few shots. The black powder fouling made it difficult to load them. That's where the lube in the pads came in handy.
Anyway, the plastic wads also serve another purpose. They protect your shotgun's bore.
Many of the earlier smokeless loads that didn't use cup style wads caused barrel erosion. The streaks looked like deep scratches down the bore. Some shot is very hard and easily marks the bore as it is compressed against the sidewalls during firing. I can only begin to imagine the damage steel shot would do.
Is it possible to use a plastic wad without the crush spring on the bottom??? Yes. As long as there is something to keep the powder and shot separated and provide a piston to drive the shot down the barrel. If you're planning to cut off the bases of the shot cups and fill the remaining space with more shot, be very careful. Pressures will likely go beyond what your shotgun was built to handle.
If you are planning on leaving an airspace above the shot, likely nothing will occur but your shot patterns may vary.
If you're planning on filling the space below the shot cup with more powder, expect something disastrous to happen.