...but I'm sure you...   =:o)

hmmm.... There should be some english in it now... =:o)

As far as I know, you must use the J/Direct or the P/Invoke to access
unmanaged
code. Or get a .net library from a third part.

Here is an example. It's in C#, but I'm sure you can translate it into J#.
(Let us know if you can't)
http://www.codeproject.com/csharp/csppleds.asp

Here is a lib for the serial port:
http://franson.biz/serialtools/index.asp

Here is part II:
http://www.codeproject.com/csharp/cspplcds.asp