It's a standard theorem in complex analysis that if $z_n$ is a sequence that goes to infinity, there is an entire function taking any prescribed values at the $z_n$. [There is a function $f$ vanishing to order 1 at each $z_n$][1] There is a function $f$ vanishing to order 1 at each $z_n$ (for $z_n=n$, you could take $f(z)=\sin \pi z$), and then consider $\sum_n a_nf(z)/(f'(z_n)(z-z_n))$. This may not converge, but you can tweak it by multiplying each term by something that is 1 at $z_n$ (eg, $\exp(c_n(z-z_n))$ for $c_n$ chosen appropriately) to make it converge.
(I don't know off the top of my head how to choose the $c_n$; this is copied from Exercise 1 on page 197 of Ahlfors's Complex Analysis.)
EDIT: It's easy to show that such $c_n$ exist. If you write $b_n=a_n/(z_n f'(z_n))$, then for any fixed $z$, the terms of the sum will be approximately $b_n \exp(c_n z_n)$ for $n$ large. You can obviously pick $c_n$ so that this converges. [1]: http://en.wikipedia.org/wiki/Weierstrass_factorization#Existence_of_entire_function_with_specified_zeroes