I think that's impossible. Internal memory is part of the hardware and it's not possible to extend or append more memory from outside. And it's also not possible to use a kind of RAID driver to bind different memory drives to a single memory partition.
No matter what you've formatted external sdcard, it's always mounted via sdcardfs. sdcardfs doesn't allow to mount other partitions or directories and it's also not allowed to use symbolic links. So you have no chance to enlarge external memory with such tricks.
One thing you can do is to mount cloud space via sshfs on internal memory. I'm using EasySSHFS. That works for me as my private cloud with 3 TB extra disk space..
Yes, older Androids have on option to move apps to external scdard thus freeing some internal space. But that doesn't increase internal memory. There is no way to add 64 GB sdcard space to eg. 64 GB internal space to get 128 GB internal memory.
BTW. The Link2SD app can move any app to sdcard. There must be a second partition on sdcard. This space is used to host apps from internal memory.