Crossdressing and intersex:
Crossdressing occurs when someone wears clothes not intended for their biological gender. If someone is XXY or XXXX then there are no clothes designed for their biological gender, and everything they wear is crossdressing.
If someone is XXY and "looks male" and dresses as male, then although everyone may think of them as "male", they are something else, neither "male" nor "female" in the standard definitions. They might not have any visible signs of their biological variation, but it is still there.
If the proceeding argument is not accepted, then it seems to me that the two possible alternative definitions of "crossdressing" would be based upon "passing" (i.e., if everyone thinks that you are a woman then wearing women's clothes would not be crossdressing), or else would be based upon genitals (which would be tough to do.)