What do you mean? You download this: "https://github.com/ratson/cordova-plugin-admob-free" ??? You can't convert, whatever.
Look, here are the steps:
1. Download the plugin from here: go down below and you will find some links.
2. Extract zip file and copy and paste here, or just move it: html5/export/plugins
3. Open construct, insert admob free, make your changes and export as cordova
4. Open intel xdk and insert this link as plugin: or just write: "cordova-plugin-admob-free"
5. Build and done!
Note: Use latest construct and intel xdk.. I test it and work 100%. If someone use old versions can't help more..