1- If you are broadcasting LIVE then this a function of your software. Depending on what software you are using (SAM, OTC, Station Playlist, etc.) you can do it. If you are using WinAmp to broadcast, then you can't.
2- If you are using Auto DJ in Centovacast, go to the Auto DJ tab in the control panel and there is a setting (Crossfade length) to set the # of seconds to fade. If you don't see the setting, ask your Stream Host to enable it or change it for you.