Hello Tiago,
Firstly, you are speaking of the Financial Plans module and not the Financials Module correct? That module is not really editable and is scheduled to be deprecated in 16.2.2. It was originally brought into MUX early on and just a copy of what is in classic.
It is being replaced by the Financials module which is quite configurable. In this module, as long as your custom attribute has an api_alias associated, you will be able to add it to the cost plan details.
Please refer to this notification
Deprecation Notice
Here is information on the Financials Module
Manage Financial Plans using the new Financials Module