I convert recipes all the time for Zymurgy. It can be tedious, and it won't give you the same results, but it's the closest you can get if you make extract recipes.
Some kind of recipe calculator helps. You can do back-of-the-envelope approximations and get close, but recipe calculators have ingredient databases and do all the math for you. When I convert Zymurgy recipes, I use BrewPal on the iphone. It's cheap and does what I need.
So the first thing you need to figure out is the total number of gravity points in the batch. For that, in addition to the grain bill, you need to know either the efficiency or the OG of the beer. If neither are listed (as in your example), you can assume 75%. You can approximate the theoretical points per pound of grain at 36. So multiply the total weight of grain by 36 and by 75% to get the total points.
If the OG and batch size are listed, then multiply them to get the total points instead. You have to come up with the same number of points by using extract. Subtract off any steeping grains (crystals and roasts). You have to know how many points per pound of grain they provide to do this calculation (recipe calculator helps, otherwise assume 36 for base malts, 30 for crystal, and 25 for roast). You're left with the points you need to get from extract.
Liquid extract is about 36 points per point, and dry extract is about 45 points per pound. So divide 36 or 45 into the total points you need and that's how much extract you will use.
This assumes that you're doing the same batch size. If the batch size changes, then you need to calculate the new total points required based on the batch size and the same OG. Scale the grains in the recipe so that you're using the same percentage as in the original recipe. Figure out the new individual contributions and substitute accordingly.
Then you just need to pick the right extract to get the closest flavor. If you want to match as close as possible, try to find equivalent extracts for each mashable grain. Or if your selection is limited, just use pale extract. In most cases, you don't know what grains went into your extract, so you're guessing anyway.
Steep the grains in your water at 150-170F, rinse a bit, then add your extract and bring to a boil. You don't need to boil as long as you do for all-grain. Just as long as your first hop addition. And you don't need to worry about messing with your water. The mash is done, so you don't care about mash pH. Just use something that tastes good.
If you aren't doing a full boil, the hop bitterness won't be right. You'll need to add more.