You also need to be a bit more specific than just "BBJ". While the 737 based BBJ is certainly the most common, there are also 787 and 747-8 based BBJs out there (there may be other models as well - but I'm positive of the 787 and 747-8).
Why do you think a 737 BBJ would be a different type rating than any other 737NG? It's the same airframe (aside from a stronger wing - -800 wing structure on a -700 fuselage), all the rest is interior furnishings.