Hello,
balun is balanced-to-unbalanced transformer as I know, and if so, you just try to find unexisting thing
There are also ideal_balun and xfrm ideal components in analogLib, second one has dif input and dif output, but no center taps... Also, I believe you can build any ideal transformer with inductors and "mind" components.
P.S. I think that you can build this from two ideal baluns from rfLib (you need just to set proper losses and impedances), unfortunately I don't have rfLib library and can't check it.