Hi Keyboard Master,
The Organ (FSB) type keyboard of PSR-SX900/700, like the Organ Standard type keyboard of the PSR-S models, is not aftertouch-capable. This functionality is a hardware property, so it cannot be upgraded via a software update, so it is reserved for the FSX type keyboards of Genos and Tyros models.
However, the MIDI sound generation of PSR-S and SX models can recognize aftertouch messages (see e.g. the "MIDI Implementation Chart" in the SX900/700 Data List). So you could connect an external, aftertouch-capable keyboard via MIDI to an SX900/700.
Best regards,
Chris