Angular / Mat Table - How to create a custom table component wrapping mat table

I want to create my own table component to add a checkbox column and actions around a set of ng-container for columns.

Example

<my-table>
  <ng-container matColumnDef="someCol">...</ng-container>
</my-table>

The custom table component would have

<table matTable>
Columns local to component 
+ columns passed from the component
</table>

Unfortunately (and obviously) I lose all the context and variables. I just don't know how to get about it.

Thanks for your help.