Add the .htm at the end of urls

Hello,

I want to add .htm at the end of the urls of the questions . The plugin is Q&A.

For example :

http://www.mywebsite.com/questions/quels-sont-les-meilleurs-produits-pour-laver-son-visage

Become

http://www.mywebsite.com/questions/quels-sont-les-meilleurs-produits-pour-laver-son-visage.htm

I can't fix that directly in Permaliens of wordpress. I have to modify somehting in your code but I don't know what.

Thanks in advance