This should be possible. The Reader shows up as "removeable media" at least in Windoz Explorer. Not so shure for Mac.
But you should consider that the PDF-viewer in the PRS-300 don't allow zoom of scanned pages, or panning and scrolling of zoom PDF's.
Zoom to M or L will do a "reflow" of the text, which is pretty useable for novels, but may destroy the layout of your technical books.