Along side being able to create store products within the store tab, you can also create products while you're creating your funnels.
Navigate to the store tab and select 'Create New Product' as shown below.
Once you have selected the create new product a side window will open. The process of creating the store product is the exact as you would within the store tab.
Once you have created your product, it will be automatically added to your store as normal. So you can use it wherever you need to.