It is already there, if you want the sales of a Inventory Item to go to a particular income account then make that selection within the Inventory Item itself.
One thing I did not explain is that changing the account, as illustrated by @Brucanna, cannot be done on an invoice by invoice basis. The reason is that proper account posting is connected with calculation of cost of goods and profitability.
the inventory sales account is selected when making a sale. when making a purchase of the inventory, the account is not a sale account.
if you are not selling inventory and want to add some expense, you can select any other account while making the invoice.
There is no restrictions, you either use the default income account (Inventory - Sales) or you nominate a Custom income account - regardless of which you choose “profitability and cost of sales” are not affected