You could use the exec funtion:
(from Manual)
exec("%EXE_DIR%\\acknex.exe","samples\\terrain.hmp"); // open terrain in another engine window
(where you can insert the -sv -cl commands)
to start gamestudio itself as a client.
And then exit the old program.
You have to write a "login" GUI then , to let the
user input the ip of the server, and use this to set the
parameters for the "re startup"
----
The missing startoption for multiplayer is known and complained about
for many years.
Its another reason why you SHOULD NOT use the native gamestudio Multiplayer functions.
Better use one of the external plugin-dlls,
where you can implement this much more flexible
during runtime of the engine.