Can I take a store bought lite set with multifunction controller ie like chasing dimming in sequence and use that box to control say 4 strands instead of one?? Wire it to control individual channels? Does that make sense??
It will depend on the MF controller. Some (like the Big W units) are designed to handle the load of a few (optional) extension strings and are supplied with a plug pack that's rated for (say) 600 LEDs. Others are supplied with plug packs and controllers that are only just enough for the included string.
You don't have a location set in you profile (hint hint), so it's not clear if you're in Australia and are talking about low voltage strings or 110V/240V types.
Not sure what your options are in that case. You *may* be able to parallel the output wires of the MF controller off to another string (cut from another set). Whether the wiring and MF controller can take the extra load is another matter.