Steep the specialties. Don't worry about convertion, the grains are roasted, burnt and/or carmalized already, there isn't anything to convert anyway! If you have to do a partial boil, why not split the extract into two batches, add 1/2 the hop schedule each and combine them in the end? You would get close to a full boil that way. But I don't know, I haven't extracted in 25 years!
Man, I am looking forward to this. I will be doing a 20 gallon batch I think!