That actually is a way to edit the .htaccess but to actually set teh DocumentRoot it's a much more difficult process. You will need to impliment mod_rewrite to achieve "Masquerading".
The process itself I'm not really sure of myself but it requires deep implimentation, and, alot of pain :P You will need to do some work with your .htaccess files, since one a shared server environment, you can't use DocumentRoot or any .htaccess extension like that.
I can take a look into it a bit and see what results I can dig up, just because I am obsessive about that kind of thing. If anyone else successfully completes masquerading please feel free to share.
I may not be smart enough to do everything, but I'm dumb enough to try anything.
PS I no longer work for Bluehost, any posts from me may not be official company policies or views and should not be associated with Bluehost.