The edit mode of the admin area does not support drag and drop. You can create a metakey to select the flv file and add it to the content item. Then associate the content item id with your control and play the video.
If you are talking about the page edit mode - you can drag the BlogPosts control onto a page and you custom control below the BlogPosts control.
Another options create a user control and there wrap BlogPosts control and your control as well. In the code behind add logic for getting the flv and associate them with the BlogPosts.
the Telerik team