You can add any standard Adobe Flash file with a uniform resource locator (URL) that ends in the .swf file extension to the video URL text box on post creation and editing forms. In cases where the URL in your browser ends in .swf just copy and paste.
If the video you want to embed comes with embed code you may be able to find a Flash URL by looking for a string of characters beginning with "http:" and ending with ".swf".
URL in Page Info
Sometimes you can find a Flash URL in FireFox by right clicking, selecting "View Page Info", and clicking on "media" option.
URL in Page Source
Sometimes you can find a Flash URL by looking at the source code of the page. In Internet Explorer you can do this by selecting "view" and scrolling down to "source".
If all else fails the website has gone to lengths to hide the Flash URL or they may use another format. In either case you may still be able to find the source URL using special software.