I just made a cider and was wondering the same thing. I think I kind of figured it out but it seems like a bit of a hack. I measured the gravity of the raw juice with a refractometer and it came out to 1.053 and I'm doing a 3 gallon batch.
When adding a new ingredient the "extract" seemed to be the best choice as liquid malt extract is closer to juice than DME, grain or adjuncts (e.g. flaked oats). I set the default volume to 8 lbs (since I found online that 1 gal of water = about 8lbs) and the yield % to 14. I left the rest of the fields blank.
When I add 24 lbs (3 gallons) the gravity came out about right but my calculated batch size (in the main recipe section) was 2.037 gallons. To fix this I created a new equipment entry in which I set the batch size to 3 gallons and put in 0.963 gal the final top up water field.
As you can see this is a little goofy, but at least now I've got the right batch volume and OG of the juice with which I can start adding on other sugars, yeast, etc to get an accurate picture of the final recipe.