no, unfortunately c# does not allow exporting of c# function calls to non-.net programs.
there are several ways you could do it, but you need an intermediary piece of software.
tradestation allows you to call a dll that supports exporting of functions, usually these are c or c++ dlls but they could be anything (delphi, fortran, whatever).
you could build an exe in c# that took functions as an argument, then you could build a c++ dll that called this exe, and call this dll from tradestation.
you could also re-write your code in a language that allowed dlls that supported extern calls (again such as c++), and then just call this dll directly.
you can see source code examples of this on the tradelink site, as well allow people to send and cancel orders from tradestation for any of the brokers supported by tradelink for free.
http://tradelink.googlecode.com