well... you could try and use includes;
e.g.
include("http://www.sub.domain.com/index.php"); or whatever but that must be setup on the server, allow_url_fopen=On in your PHP.ini config.
you could use frames to do this with the links on the side or something and the frame within the middle, this would appear that it's coming from www.domain.com and not www.sub.domain.com.
to save yourself the bother of messing around like this, just ask your server admin to enable 'url masking', alot easier, hehe
Hope you get what I mean 🙁
HTH.