android improve internal storage
You need to change the layout of the device 

You need to build a specific rom  and a specific recovery to adopt the sdcard 


  What do the specific rom  and specific recovery means ?


Let me start by how android manages storage 

As per kitakat the storage is determined by the fstab and the storage-list.xml

This define what blocks should be used for what purpose more specifically they determine the partition 


  To adopt the storage 



You need to build a new rom  with modified fstab and
storage-list.xml  (till lollipop because android 6 uses only the
fstab)
Then build twrp (or any other recovery) with modified fstab and
storage-list.xml
Then flash twrp and wipe everything using advanced wipe a word of
caution this will wipe your sdcard


Enjoy