Casting on

How do you cast on in the middle of circular knitting?

Use the knit or cable CO and your working yarn. Move the R needle to the left hand, CO, move the needle back to the R hand and continue knitting.

It depends on how many stitches you have to cast on. If it was quite a few I would do what Sue said, if it was only a half dozen or so I would probably just use a backward loop over the right needle.