Get data from a Doctype into another Doctype's table

Hi there!
I have a question.
I’m newbie so I have read the manual but can’t figure out this thing.

ERP next with customized Doctype.
I have an Item and want to add some connected Items in a Table.
I have created a sub Doctype and addedw to the Item Doctype.
Now I want to select and add an Item in my table integrated in my custom Item Doctype and how can I get data into the sub table from an existing Item example item_group, etc. ???
Custom script?
How to do that?
Please let me know.
Thank you.

Perhaps this can help you.