Thanks dnsmonster, do you know how I would do this in WHM? I am a newbie

Also, what would I recode the .htaccess file so that I don't have to change the url each time, example:
ErrorDocument 400
http://www.mydomain.com
ErrorDocument 401
http://www.mydomain.com
ErrorDocument 403
http://www.mydomain.com
ErrorDocument 404
http://www.mydomain.com
ErrorDocument 500
http://www.mydomain.com
ErrorDocument 400
http://www.mydomain1.com
ErrorDocument 401
http://www.mydomain1.com
ErrorDocument 403
http://www.mydomain1.com
ErrorDocument 404
http://www.mydomain1.com
ErrorDocument 500
http://www.mydomain1.com
ErrorDocument 400
http://www.mydomain2.com
ErrorDocument 401
http://www.mydomain2.com
ErrorDocument 403
http://www.mydomain2.com
ErrorDocument 404
http://www.mydomain2.com
ErrorDocument 500
http://www.mydomain2.com