Suppose $u$ is a harmonic function of a domain $\Omega\subset \mathbb{R}^n$ and $\partial\Omega$ has an open smooth portion. Can $u$ be extended to a harmonic function outside this smooth portion?

I have a very vague claim that if this portion is analytic, then we can extend $u$ by schwarz reflection principle. But I don't know anything about the smooth case. Can anyone give me a hint?