Can I Modify Product Import Process to Add Custom Data to Products?

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I am building a Shopify App that will allow merchants to map their products to a table that will show if their product is tax exempt or not. 

I would like to modify the Product Import feature so when someone loads their products I can have them process through a mapping procedure and then assign a custom ID tag. 

Does anyone have experience with this and can they give some direction? 

Thank you.