An obvious solution would be to use the "SendKeys" method, which can be used to "type" ALT+W and N, but I haven't been able to make it work. I'm not sure whether I'm making some mistake or that Cardbox simply doesn't respond to SendKeys.
There is however a script language, similar to VBscript, that is called Autoit3 (www.autoitscript.com). You can use it to script any application and even Windows itself. It is very easy to use, you can merge the commands with the Cardbox macro language, it's very small (± 123 kb), well-documented, and it's free. I managed to make a new window in Cardbox using a one-line Autoit3 script: Send("%wn").
Autoit3 scripts can be compiled to .exe which I did with the above mentioned oneliner. You can either download Autoit3 yourself and have a good time doing all the things Vbscript can't do (recommended), or you can download my "newwindow.exe" using the following link:
http://avs.hmr.rotterdam.nl/jamie
Make a Cardbox macro launch the newwindow.exe and your done.
Charles