Hi Ralvez,
Actually i didn't thought of the scenario that the SSD is already preoccupied with an SSD... so I went out in the lab to validate - and you are right. There 100G base SSD is present in the slot.
You could technically exchange this to a bigger SSD and do a USB installation to handle partitioning and formatting of the new SSD, but it will not be officially supported.
Strange that the base SKU is provided with a 100G SSD and you can buy an "upgrade" to 120G. A 240G version would make way more sense.
Anyway, now you know your options.