Convex combinations of weak*-convergent sequences and the Mackey topology

A Banach space $X$ is said to have property (K) if every $w^*$-convergent sequence in $X^*$ admits a convex block subsequence which converges with respect to the Mackey topology. We study the connection of this property with strongly weakly compactly generated Banach spaces and its stability under subspaces, quotients and $\ell^p$-sums. We extend a result of Frankiewicz and Plebanek by proving that property (K) is preserved by $\ell^1$-sums of less than $\mathfrak{p}$ summands. Without any cardinality restriction, we show that property (K) is stable under $\ell^p$-sums for $1<p<\infty$.