If you want to import FLV to iTunes to play on iPod, iPhone, you can refer to this detailed guide to convert your FLV videos to iTunes with ease.
part 1. Download FLV from YouTube, myspace
FLV stands for "Flash Video", which is a container file format used
to deliver video over the Internet using Adobe Flash Player, offering
high rates of compression yet ensuring the quality. It is a format that
is designed for web video playback, Notable users of the Flash Video
format include YouTube, Google Video, Yahoo! Video, metacafe,
Reuters.com, and many other news providers.
Download YouTube Videos with Firefox Plugin - Video downloader. It
is a quick and convenient way to extract your favorite YouTube videos
using this add-on. Besides YouTube, this plug-in supports other video
streaming sites.
Part 2. How to import FLV to iTunes?
There are many FLV to iTunes converters currently available on the Internet, you can choose a suitable one. I recommend Wondershare FLV converter, which performs as powerful FLV to iTunes converter, can help you convert flash video FLV to iTunes MP4, MOV, MP3 files and import FLV to iTunes with high speed and excellent quality. Now I will show the process of conversion.
It is recommended not to do other operation while converting
FLV to iTunes, as it will affect the output quality of the converted
video.
First, install and run this FLV converter, now we can finish the conversion in simply 4 steps:

Step 1: Add FLV files.
Import the FLV files by clicking "Add"
button, or just drag & drop files into "Getting started" table. You
also can add several FLV files at a time.
Step 2: Edit original FLV files.
If you like, you can
cut, merge, crop, adjust effect, trim, and add watermark to your
original FLV files before converting FLV to iTunes, to get a better
output effect. You can compare by original preview and output preview,
as shown below:

Step 3: Choose output settings.
Choose MP4 or MOV as output
video format, or MP3 audio format, all of them are supported by iTunes,
iPod, and iPhone. Also, you can customize the output parameter by
clicking "Settings" button.
Step 4: Start conversion.
Click "Start" button to convert FLV to iPod files. All the tasks will be fulfilled in high speed and ideal quality.
Part 3: Transfer converted FLV files to iPod/ iPhone
First, launch iTunes, click File option on menu, and add your converted
files to iTunes library. Then, connect your iPod/iPhone to computer via
USB cable, iTunes will detect your iPod. After that, click iTunes "File
-> Sync iPod/iPhone", and the files will be transferred to your
iPod/iPhone from iTunes library.
And now, we have successfully imported flash video FLV to iTunes, and we can play them on iPod, iPhone with pleasure.
From: Convert FLV to iTunes