Technically, yes, although I'm not quite sure anyone's ever done it (or at least publicly bragged about it ;p).
Since, admittedly, you're not going to re-read (much) of your old stuff, you can do what I did: just leave it alone, and periodically sync the stats from KOReader to Nickel.
I do that mostly for Calibre's sake (tracking read books), with
this particular Python script, which you can run from within KOReader (requires my
KoboStuff Python distribution).
Progress state is the only thing I'm interested in, but one could probably poke at more stuff.