Child Table from existing doctype

Is it possible to create a child table from an existing doc type.
I should be able to use the child doc type both as a child table as well as a single document.
Now if I convert a document type to “Is Child Table” I won’t be able to use it as a single table.

For eg.
Make Therapy Session document type a child table of Therapy Plan.
Should be able to update individual therapy sessions when the session is complete.