If you cast on at that end you’d have to pick up stitches. If it’s at the end of your work, just keep knitting. Either way you’d be making a ruffle, by increasing in each stitch across, work a purl row, increase in each stitch again on the next row and repeat until it’s the length you want. If that’s too much ruffle, increase in every other stitch instead of every one.
You can make a ruffle the other way. Just cast on twice as many stitches as you need for the body and knit until the ruffle edge is as long as you like – then knit 2 together across the row until you have half the number of stitches and continue working the body up to the neck.