I was just randomly googling and came across this post by Plush It on adding seam allowance in Illustrator. She suggests using Object/Offset path- Round and 20 px. I stuck with the round but with sharper edges you will want to use miter. Then instead of 20 px, I subbed in .5 in (which is 36 px) and it created the perfect 1/2 in. seam allowance. Score!