We present here the sensitivity study of the rare decay modes $B^{+} \to D_{s}^{+}\pi^{0}$ and $B^{+} \to D_{s}^{(\ast)+}\pi^{0}$, at Belle using a simulated sample. The {\sc BaBar} collaboration has measured the branching fraction $\mathcal{B}(B^{+} \to D_{s}^{+}\pi^{0}) = (1.5^{+0.5}_{-0.4} \pm 0.1 \pm 0.2) \times 10^{-5}$ with a data sample of $232 \times 10^{6}$ $B\bar{B}$ events collected at the $\Upsilon(4S)$ resonance. The combined data from the Belle and Belle II experiments is five times larger than the previously used. This will allow us to provide an improved measurement of the branching fraction for these decay modes, for a better bound on the ratio of the amplitudes $r_{D\pi} = \dfrac{A(B^0 \to D^+\pi^-)}{A(B^0 \to D^-\pi^+)}$ (assuming SU(3) symmetry), which is important input to extract weak phases in the time-dependent measurement of $B^0 \to D^\pm\pi^\mp$ decays. \\
In addition, we aim to search for the $B^{+} \to D_{s}^{(\ast)+}\pi^{0}$ mode, whose upper limit is provided by the ARGUS collaboration based on $209 \times 10^{3}$ $B\bar{B}$ events.