To me the structural rate of unemployment was defined by $$\frac{1}{1+m} = F(u_n,z)$$ where $m$ is the markup and $z$ a catch-all variable. We could equivalenty get this in terms of $Y_n$ by letting $u_n = 1 - \frac{Y_n}{L}$, $L$ being the labor force.
Let $L$ now rise. What happens to $u_n$ and $Y_n$? From the equation, if $L$ rises but everything else must remain constant, then $Y_n$ must rise in order to still satisfy the equation, by the same amount. But then $u_n = 1 - Y_n/L$ is unchanged? Does this make sense? Why is the natural rate of unemployment unchanged when we just increased the amount of people willing to supply labor?
EDIT: The markup is the one that firms use to set prices, with respect to wages, $\text{prices} = \text{wages} \cdot (1+m)$, F is a function which is negative in $u_n$ (more $u_n$ means smaller $F$-value given constant $z$) and positive in $z$, and $Y_n$ is the structural rate of output.