When you have a need to forward your domain name to another – for whatever reason – the easiest method is to use the .htaccess file. Yes, you have to have the write permission for the file, but any web authoring person should have this right (or don’t do it!).
Let say that you want to forward www.mydomain.com/a (but not www.mydomain.com/b) to www.a.com you can simply add:
Redirect 301 /a www.a.com