I think some pictures might help clarify...
Here's an example of what I have done
(following in the footsteps of those who have gone before!)
Maybe you can indicate if this is somewhat in line with what you want to do.
(don't get put off by the drop bars I'm using, you can do this with flat bars also...)
Basically I took the upper section of the handlepost on my Dahon Helios and cut off the top of the post, just under the welding. To this top-section (now tube), I then mounted a standard road stem (the horizontal piece at the top with all the bolts and the "O"val Concepts logo. The top-section/tube has a 1 inch diameter, so unless you have a stem that fits a 1 inch steerer tube, then you need to use a 1 1/8 shim in order to mount the stem securely These are common and sometimes will come with your stem, but if not they are pretty cheap.
Depending on what you want to do, you need to choose your stem appropriately. If you want to ride with typical mountain-bike/flat-bars (which come by default on nearly every Dahon), then you need a stem with a 25.4mm handlebar clamp. Road handlebars are typically 26.0mm, and the new over-sized road/mountain sizing is 31.8. In my image, I am using a stem with a 26.0 clamp (70mm length btw) that holds my Specialized 26.0mm drop-bar. You can often use shims to make one stem fit another size, but it is best to just get the right size in the first place.
In case you are curious, until I move to brifters/ergo-levers, that is my original grip-shift mounted below the stem using a Minoura SpareBar accessory mounting bracket.
Anyway, maybe that helps a little?
Another option is to use a quill-stem adapter in place of the hacksawed top-section handlepost. You would probably want a 1" to 1 1/8" model, the 1" would go in your bottom section handlepost and the 1 1/8" part is where you would clamp a stem to. I would just be concerned about the insertion depth, and also about twisting. Some newer Dahons have a groove in the top-section handlepost that mates with a "lump" inside the bottom section of the handlepost. This keeps the handles from twisting - if you want to fit something else in there and you have this setup, you'd need to file down the lump I think.
The other option is that you should be able to fit a quill-stem in place of the top-section of the handlepost. This is basically a combo of a stem and usually 1" or 1 1/8" tube that mounts in your frame and normally secures itself with a wedge bolt thing. Again I would be concerned about minimum insertion depth, ride height, twisting, etc.
PS. you can always buy an extra top-section handlepost from say ThorUSA
if taking a hacksaw to the only you have concerns you. They're cheap - just specify if you need one with a groove or not.