convert any type of video into .flv format

You will need to demux and decompress the video and then use the compression you want with a FLV compressor. This process is relatively simple. I know this because I worked on a multimedia project a while back and we used filters from sdk called leadtools that helped us in the work.
I found the following link to a tutorial that you can use with some modification to convert to FLV:
http://www.leadtools.com/help/leadtools/v175/dh/multimedia/to/leadtools.topics.multimedia~leadtools.topics.multimedia.creatingamultimediaconvertapplication.html
Hope this helps.

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