Loading external model in sequential updating

I am new to PyMC3. I am specifically doing sequential updating of priors.
My problem is very similar to the tutorial:

With the exception that the model in question is an external
function that I am importing from an external script. There
is a demo on loading an external likelihood function:

but not an external model. Would it be possible to get some help
on this? Are there any wrapper classes available for loading any external
model as a ‘black box’?