Any data present in the "res" folder is not writable. You can only read data from it. You can never write data to res folder on the fly. If you are looking for a way to store username and password credentials, you can make use of Shared Prefrence
Here is an example on how to use it.
http://marakana.com/forums/android/examples/63.html
Edit 1
Storing data in Shared Preference is persistent unless user clears the data using "clear data" button in your settings page.Navigate to settings->manage apps->your app. Here you will be seeing uninstall button and clear data button. And one more thing in android is you will never be able to save a persistent data. You can't use any data storage methods like preference, sqlite or file system to store a data for permanent. If user wants to wipe the data he clicks on "Clear Data" button and your data are gone. So you have make your coding in such a way to handle this.
Since this is not possible, you could try to use your app's resources which is write protected and not possible to write to it. So it depends on user or you might have to use your server to store the data over there.