You shouldn't add new fields to InventDim table unless absolutely necessary. It's pretty difficult and Microsoft prepared fields InventDimension1 - InventDimension12 for custom dimensions, therefore that's what you should use. And if you meant fields that don't represent dimensions, that would be a bad idea.
Regarding InventSum, adding extra fields is technically possibly, but it's rarely a good idea. It's quite difficult. Also, don't forget that it's a performance-critical table.
If you explain your business requirements to us, we might be able to suggest a feasible design.
By the way, I moved your question from Dynamics AX Forum to Dynamics 365 Supply Chain Management Forum.
