SAP is available in a nonlinear version. That's probably your best option. Unless you have a very limited need for nonlinear analysis, it is a pain to emulate nonlinearity the old-fashioned way by using applied loads and moments at plastic regions.
Hi Shahidi,
I'm a SAP2000 user and unfortunately the nonlinear capabilities of the program are restricted to time history analisys. In static analisys, the nlink elements have a linear behavior.