Supposing you get a request
http://subdomain.mydomain.com/user/transfer/index.php?action=register
and want to treat is internally as
http://subdomain.mydomain.com/user/transfer/transferregister
then you should place in .htaccess:
RewriteEngine On
RewriteBase /user/
RewriteCond %{REQUEST_URI} ^transfer/index\.php$
RewriteCond %{QUERY_STRING} ^action=(.*)$ [NC]
RewriteRule .* transfer/transfer%1? [R=302,L]
If you want the opposite, that is, you get a request
http://subdomain.mydomain.com/user/transfer/transferregister
and want to treat is internally as
http://subdomain.mydomain.com/user/transfer/index.php?action=register
then you should place in .htaccess:
RewriteEngine On
RewriteBase /user/
RewriteCond %{REQUEST_URI} ^transfer/transfer
RewriteRule ^transfer/transfer(.*)$ transfer/index.php?action=$1? [R=302,L]