  The extraction of the $B\to K^*$ transition form factors from lattice data is
studied, applying non-relativistic effective field theory in a finite volume.
The possible mixing of $\pi K$ and $\eta K$ states is taken into account. The
two-channel analogue of the Lellouch-L\"uscher formula is reproduced. Due to
the resonance nature of the $K^*$, an equation is derived, which allows to
determine the form factors at the pole position in a process-independent
manner. The infinitely-narrow width approximation of the results is discussed.
