If you're trying to use command-line I/O redirection operators, then perhaps you might want to have the command processor take care of it? Here's a minor variation on Tore's code:

#DEFINE SW_HIDE             0

DECLARE Long ShellExecute IN "Shell32.dll" ;
	Long hwnd, String lpVerb, String lpFile, ;
	String lpParameters, String lpDirectory, Long nShowCmd

lcOldDir=Fullpath(Curdir())  && remember original folder
MKDIR C:\TEST                && Make sure directory exists.
lcDir = "c:\test"            && directory in which command is executed
lcCmd = GetEnv("comspec")
lcPrm = "/c echo hello >> mytest.txt"
=ShellExecute(0, "", lcCmd, lcPrm, lcDir, SW_HIDE)

cd (lcOldDir)

Basically the above should be similar to:
CD \test
RUN echo hello >> mytest.txt