A slide out will require a different structural engining than a regular trailer.

I would suppect it would be a major undertaking to change the frame to support a large 6-8 foot opening in the side of the trailer.

I don't think I'd consider this an aftermarket add on. It something that needs to be done when the trailer frame was originally designed.

But, I'm not an engineer nor a designer of trailers, so maybe somebody will enlighten me.

I thought the same way as PH until I visited the Showtime and Harmer conversion facilities. Regardless of brands or models, a large hole is cut into the side of stock trailers to have the slide outs installed. An additional supporting header is added to the new roof and siding edges, while the rest of the framing was essentially left stock. Track hardware and sliders were installed onto the flooring, which supports the slide out unit. Not much technology was used, other than some good metal cutting saws.

Gard

I have owned 3 trailers with slides. One was a Sundowner, one was an Elite/Outlaw LQ and one was a Sooner/Duster LQ.  All three had heavy reinforcement beams on the bottom rail directly below the slides but not sure about the top rail since it would have been hidden out of view. I don't think I would want to pull a trailer that had a slide put in it  that was not engineered from the get go. My first slide had engineering issues that caused the double deadbolted mudroom door to just pop open when making a hard right turn. Since it was a SD, it was all done by the same folks so no one could blame the other.