Field multi-select based on custom list (not link)

I’d like to create a multiselect table.

I understood how to create a child table with a Link to an existing table. With this method I can choose among a list of name that exist in the linked table.

I would like to choose among a custom list that are not related to a table.

I first thought to create a Child table with a simple select field instead of a link field, but it is not allowed.

Is it possible ?