Converting recipes is pretty easy.
Since wheat DME tends to be a blend of 35% wheat and 65% pale malt, that would be a good choice.
Generally, 1 pound grain = .6 pound DME.
You'd keep the steeping grains the same.
So:
1 pound crystal 20L
.5 pound carapils (I'd leave this out- you don't need it with the wheat and the crystal)
6.6 pounds wheat DME (or go with 7 pounds if you're only wanting to buy two bags plus a one pound bag)
Steep grains and remove and throw them away. Bring liquid to a boil and add 1/2 of the extract. Add hops as indicated.
After adding the last hops, add the rest of the DME and whisk it in (take the pot off the heat for this) and bring it back to a boil and then turn off the flame.