How to use an existing model as in input into a new model
This depends a  bit on the used case but you could also create the 2nd model independent and then use a third model to combine them. This has the advantage that there should be no leakage between the two models (if variables are independent) and you can merge them in a very controlled way. 

What is unclear for me is what advantage you would have from taken input from a model that can not be used in the same context. (offline vs real time)

If you want to use them combined and Model A as input for B you would need to guess what the output for Model A is.