[DCP-o-matic] Trimming an existing DCP

Hello! when I have a DCP, made with DOM, and just want to trim a little part at the beginning, can it be done without rendering the picture again? I tried it with the same project and just trimmed the beginning, but DOM just starts rendering again. When I make a new project and import the DCP all i get is the folder with the xml files of the DCP without picture and sound. Am I doing something wrong? Thanks for the help Andreas