knitting question

I posted this already, as "If I start now, I might be done by October," and I put it in the knitting topic, but since I've had no replies I think people might not have seen that it was a knitting topic. Here it is again:

So, my plan: I want to knit myself a sweater. However, sweater weather here is pretty much over, and by the time it rolls around again, I expect to be seven or eight months pregnant (if all goes well, and depending on when we have our first chilly nights, whether I visit anyone up in the mountains, etc.). So I looked around online for maternity-sweater patterns, and all of them only go up to about size 20. That's fine for pants, but on top I am more like a 26. How do I modify from 20 to 26? I am probably going to work off of this pattern. It has numbers in parentheses for each increasing size up to twenty, but I don't know where and how many stitches to add to get to twenty-six, where to stay the same, etc. I have a pretty long torso, if that makes any difference.