This does not compile, any suggestion appreciated.
...
List
Compil
You can't because List and List are not in the same inheritance tree.
You could add a new constructor to your List class that takes a List and then iterate through the list casting each Object to a Customer and adding it to your collection. Be aware that an invalid cast exception can occur if the caller's List contains something that isn't a Customer.
The point of generic lists is to constrain them to certain types. You're trying to take a list that can have anything in it (Orders, Products, etc.) and squeeze it into a list that can only take Customers.