How to load/include projects from submodule Build.scala

Question

If I want to to combine two (or more) sbt projects (e.g. include one in the other) without publish-local.

folder structure:

/ComboProject
    build.sbt
    /project
        Build.scala 
    /Project1
        build.sbt
        /project
            Build.scala #this includes lazy val p1
    /Project2
        build.sbt
        /project
            Build.scala #this includes lazy val p21, p22, p23, p24

The nested Build.scala gets ignored by sbt, however do they contain the project description and more importantly the build settings. How can I import/make them visible at the ComboProject level without copy-pasting them into the top-level Build.sbt.

The same problem appears when including once project inside the other one.

/Project1 #make Project2 modules visible here
    build.sbt
    /project
        Build.scala #this includes lazy val p1
    /Project2
        build.sbt
        /project
            Build.scala #this includes lazy val p21, p22, p23, p24

Thanks a lot, this would make it a lot easier to depend on fast evolving dependencies.