Having never done steeks before, I think that you could cut right down the middle after having reinforced the edges. You could then pick up and knit a button band on either side of the opening. You might want to take a look at this to get the idea of how to use steeking to make a cardigan, though I think you'd want to use a sewing machine rather than a crochet needle since acrylic isn't as "sticky" as wool and would undoubtedly require more reinforcement to keep from unraveling.
P.S. You probably want to practice with swatches before even touching the sweater.