If you add Secondary entry (Direct Entry) in item field/ unit/ account names etc. during entry of sales or purchases voucher it is better and easy to us. Means new item/unit/account etc. will be directly entered in their respective field during sales or purchases voucher entry.
These things must be defined before they can be used and cannot be edited in a transaction form. If you want to learn how to open a second Manager window where you can make such additions, search the forum. It has been discussed often, including earlier today.