You can check Leawo Free YouTube Downloader to do that.
It can download videos and convert the downloaded videos to other formats.
This is the user guide - http://www.leawo.com/youtube-download/user-guide.html

Of course you need a downloader to help you to reach this.
i recommend you have a trial of Video Download Studio.
Download, convert and save videos directly from Youtube and more. Formats AVI, MPEG, MP4, 3GP, WMV, MP3, FLV, MOV, WAV
why not get a free trial from http://www.downloadvideos-convert.com/download-videos-music
hope it can help you more or less!