How do you cast on to a row you are already working on - not to start knitting but to increase?
You’d want to do a knit or cable CO. Check out the vid in the ‘Cast-on’ section. You’ll be adding the add’l sts onto the left ndl using your existing sts.
Sometimes a simple backward loop cast on is a good one too. Different situations call for different cast ons, or you have options.