Manually unless you are converting a ready-made sample instrument from a different format. The MPC can read AKP and 50k format programs (in the MPC software, however I've not tested this in standalone), otherwise there's various commercial conversion software out there that can convert say, Kontakt to XPM (or at least to AKP).
But be aware that the MPC Keygroup specification is very basic (4 layer limit, cannot mix round robins and velocity switching, no crossfading between keygroups or layers, limited LFO and envelopes etc). Plus memory is of course limited in standalone so there is definitely an art in creating a decent, usable instrument in the MPC, it's not just about converting from a different format, lots of decision making and compromises required.
