In order to get these non-looping clips to loop seamlessly, we need to set a cross dissolve transition between them.

Click the Transitions button to bring up the Transitions Pane. Select Cross Dissolve from the list. Set the speed of the dissolve to match the length listed in the table at the start of our tutorial for your particular clip. For our clip we set a 1 second dissolve length.

After selecting your transition settings, apply it by clicking and dragging it down to the area between the two clips. After it renders, play the area over your transition to make sure it suits your fancy (whatever your fancy may be).

If you want to place a long title over the newly lengthened motion graphics clip, you will need to export this clip as a separate QuickTime movie and the reimport the longer clip. Then place your title as you normally would

If you just want to use your newly lengthened motion graphics clip without embellishment, just start placing more clips at the end of the timeline and let your movie take shape.