Welcome to the Community - thank you for your question!
We would advise to leave feedback under the cog wheel within your account - this will submit your suggestion of being able to assign a category to a supplier directly to the developers, who will then look into implementing this in future updates if possible. You can check our product updates page for any features announced in the future: https://quickbooks.intuit.com/uk/product-updates/
Thanks,
Talia
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