First I'm sorry, I don't speak english well.
The "/dcc chat" command have no minimize parameter, but in the "mirc.ini" file (in the mirc's root folder) you can modify the ligne starting by "wchat=" in the "[Windows]" section.
More informations at :
http://www.mishscript.de/help/mircini/misc.htm(Search fot "mIRC Windows" in the page)
Warning, this will affect all yours dcc chat windows !