Yes, captions can be added to a video presentation that has already been published to make it accessible to people who are deaf or hard of hearing.
The first step in adding captions to a video is to transcribe it, or obtain a transcript if one already exists. The second step is to synchronize the transcript with the video. There are various tools and techniques for doing this, depending on the media type.
For streaming media, each of the three most popular streaming video formats (Quicktime, Real Player, and Windows Media) support closed captions, and software is available for little or no cost to facilitate the entire process of adding captions. For additional information, see the AccessIT article, How do I make multimedia accessible?  and the DO-IT Knowledge Base article, What are some guidelines for creating attractive and functional open or closed captions? 
Adding captions to analog video formats, such as VHS, is a more specialized process, and there are several service providers that do post-production captioning. For more information on locating vendors visit the DO-IT Knowledge Base article, Where can I find vendors who provide captioning services? 
-  How do I make multimedia accessible?
-  What are some guidelines for creating attractive and functional open or closed captions?
-  Where can I find vendors who provide captioning services?