Adoptable storage prior to marshmallow? As most people know, adoptable storage is a feature that allows users to use an SD card as internal storage, including the saving of apps (Similar to the app2sd system, which was removed in KitKat). Now, is there any way (Short of creating my own custom ROM/kernel) to utilise this feature on phones and tablets before Marshmallow?

Edit: I should clarify that I mean anything that replicates the behaviour of adoptable storage (so anything like editing vold file still counts, from the accepted answer, for example)
This is the closest I could find that matches OP's expectations and should work, requires rooted device ( not tested by me). This only  replicates adoptable-storage behaviour of Marshmallow to the extent it allows use of external SD to be used as internal SD for moving apps

Sources: 


Swap internal storage with the SD card in Android Lollipop and KitKat
How to swap internal storage with the SD card in Android device


Both these are blogs by the same person but take  different approaches:

Source 1 requires 


Xposed module XInternalSD :



  This module changes value (path to internal SD card) in some Android APIs which apps use. Control this path in module's settings. You can enable this module for all apps or only for specific apps. Also you can enable full access to external SD card (for KitKat+)


And


WSM Tools app that can be downloaded from Aptoide - not available on Play Store


Source 2 requires only Xposed module XInternalSD

Approach


Swap internal storage with external SD using the XInternalSD module ( Common step for both methods)





Using WSM Tools  choose apps which you want to move External SD or move all apps (Source 1)


OR


Using a root explorer amend vold.fstab  to interchange sdcard0 with sdcard1 ( Source 2)