You will need to define the incompressibility parameter. The incompressibility parameter, D, is nothing but 2/(initial bulk modulus). If you wish to add small amount of incompressibility (large but finite bulk modulus), then adding a small value for D will do it. Note that parameter D has the units of 1/stress. So, if the initial shear modulus of the material is, say 2.5 MPa, then a value of 1e-2 MPa^-1 would be a reasonable value for D.