Get Items from other doctype in Sales Invoice Item

How can we customize the following situation in erpnext:-I have a doctype say “A” which has all the details as “Item” doctype. Now I need to create a sales invoice in which the table of items should display items in doctype “A” instead of items from “Item” Doctype.
I have tried it by changing the link to A but to no use.
Could anyone help in this ??