Yes, you can add video creatives to any live retargeting campaign IF the campaign was originally launched with linked video creative. You will be required to add Multi-Touch ads to link these new video creatives when doing so (as videos that aren't linked to Multi-Touch ads will not serve)
In other words, say you launch a new retargeting campaign and added/linked your video and Mutli-Touch creatives to this campaign. Later when you come back to this live campaign you can choose to add more videos and Multi-Touch creatives to the campaign from the campaign dash.
You are not able to add videos to a retargeting campaign that did not originally launch with linked video creative.
To launch new video creative, you can create a new campaign and attach your video and Multi-Touch creative, there.