You can actually use that feature. It's just that the option in the menues is disabled to do it directly via your phone.
Just search for how to's to expand your internal storage with your sd card via adb commands.
(You'll be left with some issues about the storage statistics, but everything else should work. Although, you should disable it when you upgrade to Android 7 if you didn't already and do the whole thing again when you finished upgrading.)
Edit: Also I don't get it why people have so much issues about their internal storage. I'm a developer and as such one I have a lot more apps with a lot more data in use than the typical normal user of those phones and I'm not even close to a limit (No, I'm not using my sd card as the internal storage, because internal storage usally is faster than a sd card)
