You can use Custom Design (DOE > Custom Design) to create the design for the specific model that you want to estimate. It will save a table script to perform the standard least squares regression analysis when you have your data. (Note that you might use Augment Design instead to start with the existing data and add new runs to compliment what you already have.)
You can also use the Degradation platform (Analyze > Reliability > Degradation) to model the dependence on temperature and relative humidity and then predict the 'crossing time' for expiry.
See Help > Books > Design of Experiments, Fitting Linear Models, and Reliability and Survival for specific chapters with detailed instructions and examples.