Member since Feb 2003 · 3864
posts · Location: Germany
XEP-0054 defines no gender field.
But all server implementations I am aware of store the completed xml document in the database without any changes. Do you can just add a new <gender/> tag for example to the vcard.
// read gender
string gender = vcard.GetTag("gender");
// rsave gender
vcard.GetTag("gender", "male or female");