I have a functioning MIP model in CPLEX/OPL for production planning that generates a weekly plan of what finished good items to produce on which production resource using bill-of-materials, set of resources, resource-product eligibility, resource run rate per hour and other constraints etc.
Currently the model uses a sequence of resources with specified run rate per hour to decide how to maximize production while minimizing changeovers etc.
I have a new request to introduce a variety of interim WIP storage tanks of different capacity that supports one of the job steps (for example:filling).
The key issue is the tank resource with a max capacity as a constraint being attached to an existing resource that already is being throttled by a fixed run rate per hour.
It's hard to give a meaningful answer without knowing more about your model, as Joris indicated in his comment. At a guess, you may want to add variables that represent the amount added to/removed from each storage tank in each period, along with capacity constraints and flow balance constraints (tank level = previous tank level + stuff added - stuff removed; product output from any processor = stuff going to next stage plus stuff going to tanks; product received by next stage = product passed directly from previous stage plus stuff drawn from tanks; ...).