It will be nice if you can actually find a PCIE SSD, as of today I'm not aware of any aftermarket PCIE SSDs available. If the drive in the same as the macbook Pro and the base mini has the appropriate PCIE connector, you could buy a used drive that someone pulled out of a broken Macbook Pro. The New generation time capsule is identical to the Airport Extreme but apple removed the connector on the logic board to ad your own hard drive.
If there is only one Sata/PCIE cable/connector you might be best to put an SSD internally use USB 3 to expand your stroage.