open photoshop .psb ("big") - HowTo?

Hi All,

my PanoProg just saved a file as .psb (270MB). I would like to get it opened in GIMP (... and save as .xfc; working upon Windows XP 32bit, 4GB RAM).

Checked a lot of information, but no help until now. Possible? In case of Yes, - how to do it -?

Thanks in advance,
ykono