No laptop lets you add a soldered RAM module via an expansion slot.

Not a thing. Soldered RAM is directly attached to the motherboard. Any expansion slot would be for standard removable modules (SO-DIMMs), not additional soldered ones. The whole point of soldering is to save space and cost—adding a socket for another soldered part would defeat both.

What you’re probably thinking of are laptops that have partially soldered RAM plus one open SO-DIMM slot. That’s common in ultrabooks. But that slot takes a regular stick, not a soldered module. If you want full upgradability, look for a laptop with two SO-DIMM slots and no soldered RAM.

Future you will thank you for checking.