If you wish to be able to control all domain on an account before they resolve, you can do so by creating some symbolic links. Â Lets say you have domain1.com and domain2.com which you want to be able to access. Â You'd run the following as the user through ssh:
cd /home/username
rm -f public_html
mkdir --mode=755 public_html
cd public_html
ln -s ../domains/domain1.com/public_html domain1.com
ln -s ../domains/domain2.com/public_html domain2.com
Alternatively, if you just wish to switch which domain is being used for /~username, you can do so by loggin into the User Panel -> Domain Administration -> select the active domain to use and click "Set as Default".  This will setup the symbolic link for the selected domain for  /~username.