[Margaret Thurston]

Select the clip you want to connect, select the gray bar on top of the secondary storyline, and press Command G. (To select them both at the same time, hold down the command key.)

Note: this only works if the clip that you want to connect does not overlap with the secondary storyline (if it begins after the secondary storyline ends, or if it ends before the secondary storyline begins).

This is in FCPX version 10.3.4. I know this is an old forum post, but maybe this will still be useful to someone.