Here is a link to apps that have been already run through the process to add multi window, but I suggest doing it yourself because some the the apks are older versions. [APP] Multiwindow Apps - xda-developers
The disadvantage of this method is that you will not get updates for the app. If an update is available you will have to get the new apk and run it through the process again.
Apk. files can be retrieved by using an app backup application. Most apk. files can be gotten this way, but some apps can not be backed up.