The PRS-700 has a "NXP ISP1504 Hi-Speed USB OTG 2.0 Tranciever", see Igorsk's
PRS-700 main board front.
USB OTG is "On-The-Go", it defines a dual-role device, which can act as either a host or peripheral, and can connect to either a PC or other portable devices through the same connector. OTG hosts are only required to provide a small amount of power, which may not be enough to connect to a non-OTG peripheral.
I think this means that the "hardware" part of the equation is there for attaching a keyboard to the PRS-700, but I don't see software support being likely unless OpenInkPot (say) is ported to the PRS-700.
OTG is now relatively common, but I don't know if it is on the older PRS-505.