That would, depending on the internal layout of the two machines, range from almost impossible to highly impossible.
First of all, the airport card in the ibook may or may not be soldered onto the logic card. Soldered = no.
Then, You'd have to find a port to plug it into in the imac. No port = no.
Next, There would need to be ample room to put the card in without constricting airflow. No room = no.
But, if you could do that, the imac would probably recognize it and you'd be home free. 😛😀
Just don't try, unless you're really, really ready.