With easyDCP Creator 2.0 or newer, you can load a DCP using the "Open DCP" option in the "File" menu.
Once loaded into easyDCP Creator, you can add subtitles, add a new composition or modify an existing one, change the names, etc. Provided you have not changed the essence, but only metadata, you do not even have to re-generate the DCP, but instead just overwrite the existing XML-based metadata files by clicking "Save DCP".
Similarly, you can enhance the loaded DCP by generating a Supplemental DCP.
To do that, just right-click the loaded track files and mark them to be used as references.
Referenced track files will not be included in the new package. After importing new track files and assembling a new composition, generate a new DCP. Only the new track files will be included in the generated DCP and therefore it cannot be played by itself. However, when you ingest both the original and the supplemental DCP into a cinema server, the server will resolve the referenced track files of the Supplemental DCP (VF) against the original DCP (OV).