User wants to use existing platforms with an extra flag like static without duplicating the platforms which leads them not being updated along with the existing platforms.

One way could perhaps be a field below which does thing based on the id: 

include: [cmake_platforms.yaml]
include_modifier:
  - id: CentOS-host
    configure_args: "-foo"
  - id: ".*host"
    configure_args: " -static" 
  - id: ".*"
    features: "Conan"

Additionally one could choose to prepend or append.