If you converted a 20" Dahon, you'd have to completely replace the handlepost; these are so flexy on the Dahons that even with basic road riding you can feel the flex. You'd snap it in no time. The frames are also rather flexy, and without the top tube, they will not provide the strength and stability you'd need for most tricks / flatlanding. (You might be able to grind if you replace the handlepost, but I'd lock down the frame hinge like nobody's business first.)
Any folding bike is going to take up significant trunk room; for example, the 20" Dahons fold down to 13" x 25" x 32". The Puma will probably take up the whole trunk. In fact, in terms of space a 20" folding bike will probably take up as much space as a BMX with the front wheel off. So no matter what, I'd think you are taking out the bike when the snowboard goes in.
As to couplers, if you aren't a professional framebuilder yourself, I would not DIY this project. This is not like swapping out handlebars or a BB, retrofits require a great deal of precision and rock-solid welds.
If you want couplers, your best plan is to find out which framebuilders in your area are on this list,
give them a call, and tell them exactly
what you want to do. If it can be done, they'll give you a quote. If it can't, they'll explain why with much more in-depth knowledge than what you can get on a web forum (or at least, from me.
Since this thread is 5th from the top in a Google search for "folding BMX" in less than 3 days, though, I'm going to say that folding BMX's do not currently exist, and are definitely not in mass production.
Making one might be a fun project, but really I think you will be better off buying that Haro and a cheap-ass BMX
as your backup instead.