Hi,
if you want to extend a clip that’s in the primary, all clips downstream from it (with attached ones) will move downstream accordingly, wether you extend by trim or extend function.
if you want to extend by inserting a clip in the primary, well, just insert it and all clips downstream will move
if however you want to extend by adding a connected clip over some slug in the primary, insert slug with alt-W which by default will give you a 3 second gap. You can then select it and ctrl-D to change its duration. All clips dowstream will move. Connect new shot, trim to length and then trim slug underneath so as to bring back all following clips and close the gap.
in all cases I see no need to lasso and move clips, if I understood right the problem you mention.
hope this helps
Patrice Freymond
Editor  Certified Trainer FCP7/X
Post Consultant
Always learning…
Edited to apologize for missing the whole issue of the music being in the primary….duh