Back in 1.4 it was just a trick with the fairings, I don't know if it still works in 1.5.
Basically this:
-You build all your SRB outside the ET, by clipping tanks together.
-Then you put a fairing over it and you stretch it to the lenght you want your SRB.
-You put something to make sure the fairing doesn't detaches.
-You attach the whole thing to the ET with a separator. Make sure no other part is attached to the ET, otherwise it won't detach!
-Test it.
It should work.