Can I model shear walls using plates ?

Yes you can, specially if you are interested in analyzing the wall and finding out the forces ( shears and moments ) for design of the wall. You will have to calculate the reinforcements manually. Normally you would need the in-plane shear and the in-plane moment for designing the shear wall. These can be obtained by using Results Along Line feature in STAAD.Pro which can be accessed from within the Post Processing mode. At any elevation along the height of the wall, one can get the in-plane shear ( Fxy ) and the In-plane moment ( Mz ) both of which are reported within the Total Force tab of the Results Along Line table. These data can be used subsequently to design shear wall as per any design code manually.