–custom-charsetN= (or -1, -2, -3 and -4) is not enough

[xieli9_fly]

when i crack some hash like md5, i define custom-charset, but is only 4 custom-charset is support at most, -5 , -6 is not support.
how can i expand the custom-charset to fit more complex input?

[atom]

no one yet has ever really needed -5 or even -6