We describe a perturbation method that can be used to reduce the problem of finding
the multivariate generating function (MGF) of a non-simple polytope to computing the MGF of
simple polytopes. We then construct a perturbation that works for any transportation
polytope. We apply this perturbation to the family of central transportation polytopes of
order kn x n, and obtain formulas for the MGFs of the feasible cone of each vertex of the
polytope and the MGF of the polytope. The formulas we obtain are enumerated by
combinatorial objects. A special case of the formulas recovers the results on Birkhoff
polytopes given by the author and De Loera and Yoshida. We also recover the formula for the
number of maximum vertices of transportation polytopes of order kn x n.