The new animation needs to be in a 2nd Choreography action.
>New>Choreography Action on the "Shortcut to" your character.
In the PWS, make the redbar of the first end where the second one begins
"Blend Method" on the new Chor action should be set to "replace".
The first Chor action should have "hold last frame" ON
The new chor action should be below the first one in the PWS.
set the time to the first frame of the new bar
select the 2nd Chor action. It will get the Green check on it in the PWS.
set your key filter to "Key Bone"
select a bone that you will be translating and/or rotating in the new animation and force a translate and/or rotate key on it...
Force a translate by using the left/right cursor keys to nudge it out and back.
Force a rotate key by turning on the Rotate manipulator ® and rotating the bone from and back to its current position. This is easier if you temporarily set your Options>Units>rotate increment to a high value like 30.
Do this forcing for every bone you need to animate in the new Chor action
Now you can go on to later frames, animate new movement for those bones and the keys will not affect what was in the first Chor action.
It used to be possible to CTRL- select all the bones at once and force a key on them, but a bug is currently preventing that workflow from working.