Is it possible to put a smoke unit in one of these locomotives?
If you’re handy with a drill-press, absolutely.
http://www.bachmanntrains.com/home-usa/dwg/dwgs/H508X-IS001.PDF
The new chassis isn’t designed to hold a smoke unit, but it looks like it wouldn’t be hard to drill a hole and put one in there. Just remember, smoke fluid gets everywhere, and it doesn’t look all that realistic. It is fun for a little while, though.
Do you know how I would connect it to the PCB that holds the decoder? Or would I have to get a new decoder that has that function?