  It is proved that a general non-differentiable skew convolution semigroup
associated with a strongly continuous semigroup of linear operators on a real
separable Hilbert space can be extended to a differentiable one on the entrance
space of the linear semigroup. A cadlag strong Markov process on an enlargement
of the entrance space is constructed from which we obtain a realization of the
corresponding Ornstein-Uhlenbeck process. Some explicit characterizations of
the entrance spaces for special linear semigroups are given.
