there's any command to, instead of going into login screen using system.logoff, to change from a profile to another specific one directly from a button?
From the wiki's list of built-in functions: LoadProfile(profilename, [prompt]) - Load the specified profile. If prompt is not specified, and a password would be required for the requested profile, this command will silently fail. If prompt is specified and a password is required, a password dialog will be shown.
Just add a button control to the XML file where the other power buttons are defined (I think this varies per skin) and use the LoadProfile function in the <onclick> tag/