You can use circular needles for flat knitting also, at the end of the row you just slide your work onto the cord and turn. Its easier than manipulating those long straight needles.
I think it would be easy if your sock pattern stitches are stockinette, because thats what you get with circular knitting, knit around and no turning, voila! its all stockinette stitch.
From what I've read on this subject you should check your gauge, as it may change from the purl stitches you are not doing!
If you want garter stitch with circular knitting you mark your rows and change from knit to purl at the end of every row, the exact opposite of flat knitting!