In some cases, your customer or partner will need to customize your application using extensions, but may find themselves having to extend an element that is already an extension. In other words, if you extend Microsoft metadata and your customer wants to extend your extension, will they be able to do so?
Developers can follow the procedure described in the example below to extend an extension, it works in many scenarios.
Let us assume you are working on the “Fleet Management Extension” package and want to first add a new field group to the FMRental table. The FMRental table is in the fleet management model. Here’s what you would do:
You have distributed your solution (the "Fleet Management Extension" package) to a third party that would like to extend MyNewFieldGroup, here’s what they would do.
The main thing to keep in mind is that you always extend the base element, even if you have a dependency on another extension.