I'm not quite sure if these steps will work 100%, but i think it's worth the try..
We'll be needing these 2 devices to make this storage upgrade possible.
1. A Lightweight mSATA to 18 Plus 8 Pin SSD Adapter for MacBook Pro Retina.
2. Samsung T3 Portable SSD - 250GB - USB 3.1 External SSD (MU-PT250B/AM), just for the sake of running this test, if it is 100% working flawlessly we can install the 2TB ssd version afterwards.
5 simple installation steps and you're done.
1. Install the mSATA to 18 Plus 8 Pin SSD Adapter on your MacBook pro.
2. Open/Dismantle carefully the Samsung T3 250GB SSD casing, you'll notice that the Samsung T3 SSD is attached to mSATA adapter to type-C.
3. Carefully remove the samsung T3 SSD from the type-C adapter.
4. Insert the samsung T3 250GB SSD onto the mSATA slot to 18 Plus 8 Pin SSD Adapter, the one you installed in step 1.
5. Put all the parts together of your Macbook pro, boot up and test the new SSD drive.
If it detects the drive then you can install fresh iOS on it, then you can upgrade your MacBook pro storage up to 2TB SSD at a cost of under 800 dollars.