Portfolio with liabilities (LDI)

Dear all,

I?m working on a paper that analyze the performance of alternatives investment in a liability driven framework (LDI). Basically adding alternatives investimentos to a pension fund portfolio.

My portfolio return is the return on assets minus liabilities. 

My question is: Does any know if there is an package that would allow me to add liabilities to a portfolio? Or do I need to restric the weight balance to the asset and ass in the liabilities with a negative sign?
I?m not sure how to include liabilites in the optmization.



Any help will be much appreciated,


M?rcio R. Bernardo