Adoptable storage prior to marshmallow?

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:

1. Swap internal storage with the SD card in Android Lollipop and KitKat




2. 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)