you can make a button to call new item quick entry form with that.
Or you can try below steps to verify if you have code in your database. and create from there.
when ever an item code/name entered check the database for it’s present or not
if item code/name not present in database open new item quick entry form
but it gonna be problem if you entered not well remembered code/name, so create duplicates and also if you have a lot of items in your database your queries will be slower, and it gonna be pain to enter 20+ item in an Quotation.