While there may be some way of phsyically moving the ibooks to a nook, you would not be able to read them anyway. Apple's DRM scheme is incompatible with the nook.
When you sync with iTunes, your books are backed up to your PC. You should be able to find them by checking the properties to see where they are stored.
You can then side-load the ones that don't have DRM (not all do, even in the iBooks store, I've found). Those with DRM can only be read on an Apple device.