There are many ways to make hair and wigs for dolls. You can also buy pre made doll wigs in a variety of colours and styles.
The following video shows an easy way to make yarn hair for any doll including amigurumi. In the video the hair is stitched together to join the hair line, however you can also crochet in slip stitch to join using the hair yarn.
You can also crochet dolls hair in a variety of ways to make curls and locks in a number of ways.
Or use synthetic ready made doll hair