As @DRM said. Turn on ripple. CTRL R.
Open the clip you want to add to the middle of the timeline in the source viewer.
Press C/CTRL C to copy. Place the play head at the position you want to insert the clip into the timeline. Press V/CTRL V to insert the clip and move everything right or B/CTRL B to overwrite and move everything right.
The blue clip was inserted when the play head was at the red dot.
