Saving values and adding new ones from an editable grid in an interface

Hi! I have created an interface which allows the user to create PO Lines (shown below). The information is saved in a Record Type ("CPF Lineas Pedido"):

Additionally, I have a previous interface in which the user fills "global" or "header" PO fields, like the id, supplier, total price, etc. This values are saved in a different record ("CPF Pedidos")

In my site, the user can see the record list of CPF Pedidos (the basic fields). And, when they click in one of the PO, a summary view appears with the information of the PO Lines. In order to do this, I added to the summary view a read-only grid, filtering by PO.

Now, I would like to bill each PO. For this, I created a related record action "Facturar Pedido", which initiates a process model and has another two interfaces:

1) Interface 1: Header fields of the invoice. The fields of the PO such as the PO identifier, supplier... must appear automatically (extracted from the PO information/ record type). This works fine, and values are saved in the record type ("CPF Facturas")

2) Interface 2: In this interface, I would like to bill the PO lines. For this, I need the PO lines information to appear, and I was thinking about using the same formula I explained before (using a Record List Component/Read-Only Grid/Editable grid based on the record). But my doubt here is, ¿Is it possible to add new columns and configure them?  --> all the values must be saved into another record ("CPF Lineas Factura")

¿Should I use another way to save the values of the editable grid and being able to add new columns (Invoice id, Invoice Line number...)?. I know that it is easy to save the values with text field components, but I´m not sure of how to do it with editable grids

Also, any advice or best practice to make a billing button of the PO and PO lines would be useful. ¿And could be possible to bill only one PO line, and not all the PO lines at the same time? How could I structure everything in order to do that?

Thanks a lot,

Carlos