You can get this ability by enabling the intermediary screen for product category selection before product creation.  To do this, go to the additional settings page in your OpenCRM settings. Under the "Product Settings" there is an option to 'Pre-select product category before starting edit'; check the tick box for this setting and save the settings. 

Once this is done, the next time you create a product you will be taken to a screen which will let you select the category of the product before taking you to the edit page.  The edit page will then have the product category set, and all conditional fields associated with that category will be available for adding data, without the need to save the product first.