Assuming you have access to a registered account that contains the content to which you would like to add the subtitles for translation, hover over the thumbnail of the item and hit the text balloon to open the Subtitle editor. 

The item will be displayed as follows:

Indicate the primary language of the existing subtitles and use the option "OPEN an existing SRT/STL subtitle file". A dialog will pop up by which you can select an existing file that contains subtitles. 

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