As long as you leave enough yarn on each holder to weave your ends in when you are finished, you can simply cut them.
Do you mean use a circular needle to finish the collar? You can use circular needles the same way you would use straight needles and knit back and forth. I do all my flat knitting on circular needles.
If you want to knit the collar in the round, I think you need to leave off a couple of stitches that would be used for seaming.