Copy the coding below and paste it on your .htaccess file. If you now open something.pga it will call something.htm from your server.
The reason I asked is there are different ways to do it, especially depending on how you are creating the pages
Modrewrite is not necessarily the best or easiest way to solve this, but you haven't informed as to how you are creating the pages. You can also set Handlers in Apache to handle certain extensions, probably the easiest way.
With .htaccess u can make .pga extension work as html but then u will have to change the extensions of all the .html files manually