  We prove that if an analytic subset $A$ of a linear metric space $X$ is not
contained in a $\sigma Z_\omega$-subset of $X$ then for every Polish convex set
$K$ with dense affine hull in $X$ the sum $A+K$ is non-meager in $X$ and the
sets $A+A+K$ and $A-A+K$ have non-empty interior in the completion $\bar X$ of
$X$. This implies two results: (i) an analytic subgroup $A$ of a linear metric
space $X$ is a $\sigma Z_\omega$-space if $A$ is not Polish and $A$ contains a
Polish convex set $K$ with dense affine hull in $X$; (ii) a dense convex
analytic subset $A$ of a linear metric space $X$ is a $\sigma Z_\omega$-space
if $A$ contains no open Polish subspace and $A$ contains a Polish convex set
$K$ with dense affine hull in $X$.
