Media Composer does not allow you to Splice or Overwrite Filler at the end of a sequence, but all is not lost. We'll look at two methods you can use. The recipe will look at using Trimming to add Filler at the end of a sequence, and the There's more... section will provide a second method that uses the Title Tool.
Add a new video or audio track (whichever you prefer) if you do not already have one that is empty at the very end of your sequence.
Enable only the Track Selector for the track that is empty at the end of the sequence.
If you have Sync Locks enabled, then disable all of them (or at least disable it for the track that is empty at the end).
Go to the end of the sequence.
Make sure that the Position Indicator is parked at the end of the sequence and not all the way to the right-hand side and actually parked off the sequence. You may need to move the Position Indicator back one or two frames from the very end.
Use the Add Edit function...