Are you manually packaging your APK using the ADT tool? If so, you can add resources to your package (and specify which folder they should go in to) by using the -C and -e options.
-e places a file or directory into a specific folder in your package
-C changes the current working directory on your development machine
Here is an example. This will place the file 'asset.png' into the folder [package root]/lib/assets
adt [adt command here] -e "C:\path\to\my\asset.png" "lib/assets"
You can also specify entire directories (recursively) instead of individual files.
Here is an example. This will place the directory (and sub-directory) 'fortumo_res\assets' into the folder [package root]/fortumo_res
adt [adt command here] -e "C:\path\to\fortumo_res\assets" "fortumo_res"
Or, you can place the 'fortumo_res' assets into the 'assets' package folder instead:
adt [adt command here] -e "C:\path\to\fortumo_res\assets" "assets"
You can have as many as you like. Example:
adt [adt command here] -e "C:\path\to\my\asset.png" "lib/assets" -e "C:\path\to\fortumo_res" "fortumo_res"
Hope it helps!