Official comment
If you look at the XDJ-RX2 rekordbox Hardware Diagram, you'll see that you use the BEAT JUMP pad mode as the SAMPLER mode (if you want BEAT JUMP it's SHIFT+SLIP LOOP).
When in SAMPLER mode, pressing one of the pads will play that sample, holding the shift wile pressing that pad will pause the sample. Unfortunately there is no pre-assigned method of loading a sample to a particular bank - the best function for that would be a keyboard shortcut or using your mouse or an additional MIDI controller to map that item.