we had an exact same requirement, and we landed up building our own small simple rule based module to handling product data entry jobs. We created views before getting into import export to make sure that categories, products, and order tables remain in sync.

Before you do such a straight import, there are many things to consider (a) Open orders against existing product, (b) categories, (c) price etc...; you need to run couple of validations if you want to keep your db clean for future upgrades and not to break existing functionality.

I suggest building a view which pulls at minium categories and products from _categories and _products table, then export the view output to excel file. Now this should act as your base template for import processes.

Hope that helps,

Sincerely,