Calculate R-Square for existing predictors

Hello everybody,

in the past months I have developed a model to predict the mortality rate of
Acute Myocardial Infarction Patients. My evaluation was mainly based on ROC
Curves etc. 
I have now been asked to calculate the R-Square of my model, another existing
model in regards to the actual mortality rate. So far I have only found a
function calculating a regression, and then the R-Square of the calculated
Regression.
Is there a function to calculate the R-Square for existing models?

Thanks for your help

Katrin