The mod doesn't support checkboxes (or text fields). Most attribute types allow only one value per attribute. Checkboxes allow you to have any number which would represent a huge uplift in the mod's complexity and probably necessitate a completely different approach.
However, in most of the cases that I seen where clients wanted to mix checkboxes and stock, they were actually trying to cross sell additional products and it was possible to achieve the intended results by treating them as separate products with their own stock (as they really should have been). Though I don't know whether that would apply to your situation.
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