Incorporating an SD card into Internal Storage will increase the storage and make the data on the card unreadable outside of the phone, which might sound tempting, but in terms of integrity the SD card will be the weak link by build and connection, and you will hand the integrity of the whole Internal Storage to it. If it breaks (which is not improbable), your whole Internal Storage might go down with it.
Judging by experience in this forum this would be asking for trouble sooner or later.