In STAAD.Pro one can model the concrete cross sections but not the reinforcements. The reinforcements are calculated by the software based on the forces in the structural elements. One can force the software to select the same diameter bars, as in the existing structure, through using appropriate design parameters. So for your wall, you may use a mesh of plate elements. If the existing wall is a shear wall, then using surface element would be appropriate.
Also the following post may have some useful information for you
