There are a couple cast on's you can use in the middle of a project. You can do the knitted cast on or cable cast on by turning the work around to the other side and cast on the stitches then turn it around again and continue. Every time you need to cast on you turn the work again.
You can also do a backward loop, but that one sometimes has some problems with the last stitch getting stretched out or being hard to knit into.
Up to you though. The videos are in the video section at the tab at the top of the page.