As far as I know there is NO way to "Paste Bassline to track " like in Reason. but what you can do ,if the 303's sequence is internal (written in the ABL) you can start that sequence by easily writing a note across one bar in a Ableton clip . and obivoiusly if the sequence is internal on the ABL make sure the seq to host is activated and that sequence will start when you start. Hope this helps you !? I wish it had a midi out so you could rec those notes as well.