In order to add closed captions to a StreamSpot archive, you will first need a .vtt file. If your file is the common format .srt you can use this conversion program to easily convert it to a .vtt.
To create the file you can utilize a handy (and free) tool at https://www.nikse.dk/SubtitleEdit/Online#.
To start, click Subtitle then New
Next, open a video you wish to caption by selecting Video then Open Video From Disk. (to download an on-demand archive to your computer, see: How to Download an Archive)
Navigate to the appropriate video and click Choose File.
With the video loaded highlight the selection in the timeline that you wish to caption. You can preview the video with the player controls in the upper right.
Right-click in the selection and click Insert Subtitle Here.
Type the caption that corresponds with the highlighted video segment:
Once complete select Subtitle in the upper left and then Save/download.
Next, choose .vtt from the list of available formats.
You now have the .vtt file required for adding Closed Captioning to your on-demand archive.