You can not inherit external table which is not define into Odoo Model structure.

To manage any kind of relation between any two tables using Odoo you have to define those two tables inside Model structure of the Odoo. In short, in your example 'tbl_external' must be a valid ORM class in order to extend all of it's columns into your table. If it is just a database table then it is not possible anyhow to inherit all the columns. This is said to be "Inheritance by Delegation" in Odoo. 

It is better written/explained over here in this link ( Please see deligation section )

Hope this helps,