Prediction with parameter estimates on new data

Hello NMusers, I am new to NONMEM, and I am unsure how to use parameter estimates for prediction with new data. More specifically, I have a K/PD nonlinear mixed effects model developed in NONMEM, and I want to check its predictive power in a specific way. The data I've been using has about 500 patients, each with multiple dosing (AMT) and effect (DV) measurements. Most patients have many measurements, spanning several months. What I want to do is use the population estimates (and potentially also the between-subject variability estimates) from fitting the model to my entire data set to generate the predicted effects using only early data (for example, the first 3 months). I have the population estimates from fitting the model to all data, and I want to generate the model predicted effects using only early effect data (though with all dosing data), so that I can asses the model predicted effects for later time points. I am new to NONMEM, and am unsure if there is a way with NONMEM or PsN of using previous estimates on new data, in this specific way. I am looking into using R's deSolve package, since my model contains two differential equations, but I wanted to know if there was a more direct way. Thanks for any help, Lib Gray