Merging web.configs between projects

I have a common web project which is used as a base for several "child" web projects. Is it possible to apply a web.config transform/merge between projects? Let's say the structure looks like this:

base project
  - web.config

child project
  - web.config
    - transform.config

Is it possible to make a pre build event or similar that merges the base project web.config with the child project web.config?