You can set keys on a character set that already has animation clips so that the keys are automatically placed on the curves in its current, active clip. The technique of inserting keyframes into an existing animation clip is destructive.
If you need to place many keys in a clip at a time, key into the clip from outside of the Trax Editor.
To key into a clip from the Trax Editor
Keys are placed at the current time on all the pertinent animation curves in the selected clip. If you key into a clip (original) that has duplicates (dependent copies), those keys are also inserted into the duplicate clips.
To key into a clip from outside of the Trax Editor
The clip turns purple to indicate that now when you set keys for the objects whose animation curves are in the clip, all those keys are automatically placed in the clip.
All keys automatically appear on the animation curves in the selected clip in the Trax Editor.