It all depends on the type of extract. If the recipe uses strictly light malt extract, it's a lot easier to convert. Dark or amber extracts are hard to convert because it's multiple different base malts that go into making it and it's all up to the guy who makes the malt. Good luck getting that info from the company (I tried to convert an old recipe of mine where I used dark extract and breiss wouldn't help me).
Anyways... here's the basics.
-Keep all of your specialty malts the same as the all grain recipe. You'll want to convert just the base malt.
-Assume that DME=44 PPG, LME=37 PPG. (points per pound per gallon)
-You'll want to multiply the amount of base malt by 37 (average malt PPG)
-When you have that number, multiply it by the recipes efficiency (as a decimal)
-This should give you a number that you can then divide by 44 or 37 depending on what extract you'll be using and it'll give you the amount of malt extract you need.
10lbs of base malt*37PPG=370 PPG
370*.75% efficiency=277.5 PPG
277.5/44=6.3lbs of DME.