You can use Duration command that will change clip duration to a specific length.
Also if you want change Speed of clip, you can use Speed command.
As I understand you want to change clip duration and leave in/out points intact, therefore to change speed of a clip.
You can do this with native Speed/Duration command. If you want to automate it with Excalibur (create options), I think you can create user command that will call up Speed/Duration window and type speed or duration needed.
Will it work for you or would you rather prefer Speed+Duration (where they are linked) command added to Excalibur itself?
