You can specify a window rule for the chat window. Right click on a randon window title bar, choose Configure Window Behaviour and then Window Specific Settings. Make a new rule, and "Detect" the chat window (not Kopete's contact list window). In the dialog which appears after detecting, you should select the second or the third option (at least something with the window role). In the other tabs Geometry of the dialog, select Desktop 2, either forced or initially.