How to estimate "necessary" economic output using Leontief input-output impact analysis? In particular: how to handle imports
If you want to model exports and imports you need  interregional (or multiregional) Leontief input–output model instead of standard one.
The way how these models work is that you will have separate foreign sector in your input output model, along what you can see in the table below (taken from Hewings, G. J., &amp; Jensen, R. C., 1987):

This is in spirit similar to assuming that foreign are just some other production sector - but the issue is that spending and productivity might not necessary be the same as at home. There is also more nuance to it. I would recommend going over the above mentioned Hewings, &amp; Jensen (1987) which has all the details on how input-output analysis is different for open economy.