Why create a custom APK? Why not download the content that needs to be customized?
Anyways, you can share resources from different APKs using a sharedUserId and getResourcesForApplication() (link). You would still need to make each an APK.
Some info from Roman Nurik:
Share files between applications
If your apps are signed with the same certificate and have the same
android:sharedUserId declared in the manifest, they can share data in
that they can read/write to each others' data directories. They can
even share a process if you'd like them to (see android:process).
More links to similar questions: