We had a similar kind of requirement in one of my previous projects. We used the IBM BAW local repository to upload the files while the BPM instance is in progress. However, before the process was completed, we were uploading all the documents from the local repository to FileNet using the admin credentials. And then deleting the files which are being uploaded to the local repository.
This should not be a compliance issue because during the workflow processing, the files are being uploaded by the local user (to the local repository of BPM), the local user who is the owner of the process/task; however, at the end of the process, the admin user becomes the owner of the files and uploads them to FileNet.
To allow the files to be uploaded to the local repository of the IBM BPM process, you need to enable the flag "Allow locally managed documents" in the "Folders" tab of the BPD, and in the coach view settings also need to configure the local repository instead of the external FileNet repository.