I knit with circular needles all the time even with a flat project. It seems to be more popular amongst knitters these day to do just that.
After completion of your cast on, you join them in round by knitting into the 1st cast on sts and then the rest of the cast on sts until you complete the round. Make sure all the stitches are not twisted or turn into different direction. It's a good idea to use stitch marker at the beginning of the round so that you won't get confused. It happened to me many time!
It's hard to imagine how to do that, so I've made a video for you. It's unedited, so I hope it's good and clear enough for you to understand.
Here is a video on how to join the cast on stitches together on circular needle.
If you saw only a blank space above, that means the video has not been loaded. You can use this direct link to the video; http://youtu.be/IV2-Bki5XGA
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