Yes - you can hybridize cal kings with milks, corns, etc. - I have no idea how hard it is as I've never tried. Apparently there are some tricks to increase the odds of a succesful copulation.
My understanding is that with corn snake X cal king, it is easier to use a female cal king and a male corn because of the shape of the reproductive organ.
There is a hybrid forum here where you can probably get some more information on it.
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6.8 L. getula californiae - 11 eggs (Cal. King)
1.2 L. getula nigrita (MBK)
1.0 L. getula floridana (Brooksi)
1.0 Pantherophis guttatus guttatus (Corn)
0.1 Pituophis catenifer catenifer (Pacific gopher)
3.3 Elgaria multicarinata multicarinata - 14 eggs (Cal. Alligator Lizard)