Re: access existing files stored on system
Let me make some assumptions about what you are doing.
First, it sounds like you are looking to add files to a file gallery that are already on the server.
Second, it appears that your File Galleries store files in the file system, not in the database. If this assumption is wrong, I think you are out of luck.
I don't know if you want the files searchable. If you do, then I would suggest you copy the files to your local machine and then use the File Gallery upload to upload it again. That will create the correct references in the database for the file. If you have enabled searching, it will also index the files. You could then go into the file system and remove the file and replace it with a symbolic link, but I am not suggesting this.
If you don't create a search index, then you could upload a dummy file and then replace the file in the filesystem with a link to the real file. You should also update the file size in the database.
If I have a really large file that I cannot upload (due to Apache upload limits, for instance) what I do is ftp the file to the server. Then in tw upload a fake file (to create all of the appropriate references) then replace the file that was created with the real file. Then I update the filesize in the database. I may (or may not) re-index all of the files at this point.
gary