I don't think it's possible to access the serial port using Dark Basic only, although i'm pretty sure you can create a windows dll using C++ that could give your Dark Basic application this extra functionality.
Over the years a lot of people have made plugin's for Dark Basic Classic and Pro so i wouldn't be suprised if a quick scan of the
DLL Talk forums turns up what your looking for. On the other hand if your feeling ambitious then you could make one for yourself; check out this paper on
windows serial port programming:
http://www.robbayer.com/files//serial-win.pdf