You can't join with straight needles. They can only be used for back and forth knitting. For a baby hat, you might be able to start with a short circular, but you'd have to switch to double-pointed needles or use one of the other techniques for knitting in the round.
You can, however, knit a hat and seam up the edge using straights.
"Trust the pattern!"
Time flies like an arrow, but fruit flies like a banana.
Hop aboard while we follow the cloud-covered van of knitting harmony, man!! Woooo!! --Hedgehog
IngridKH on Ravelry