i'm doin a room full of BB presently and i'd cross it with a white russian.but bubble to ,its not hard if u know wut ur doing.. this is wut to do to cross-bread;
take a plant of ur first strain.. must be female
grow the fem plant till its in its about 3rd - 4th week of flowering
grow a male plant of the other strain you want
once the male plants ballsacks open scrape the pollen out of the sacks and apply the pollen on 1 single branch of the female plant carefully
wrap that single pollenated branch in the female with a plastic bag & elastic band it at the stem coming off of the main stem of the plant(that way it wont seed the rest of the plant. once your female(first strain) reaches maturity chop it down harvest the good buds and seperate the one branch from the buds
from there u should have ur seeds! cross-bread! good luck!