What is the recipe? Unless there's specialty grains that require mashing, it's really easy to interchange. Just swap out the base malt with a calculated amount of extract. The quick, dirty ballpark way is to use ~.6 lb light DME or ~.75 lbs light LME per pound of base malt. Keep the specialty grains the same, and steep as usual.
More involved (and accurate) but requires some math:
http://***********/stories/article/indices/50-recipe-calculations/620-extract-to-all-grain-and-back
If you use brewing software, it should be really easy to just swap out the base malt with extract and use the software to figure out the right number.
And if there's specialty grains that need mashing, you could always keep a bit of the base malt in there (and use a little less extract) and do a small partial mash.