If i might add a word here.
As far as i understand the DSL reference it is not possible to do this because the only possibility to add Projects is through the include or includeFlat methods, which evaluate the file path relative to the directory where the settings.gradle reside and its parent, respectively.
But if you leave the API behind, you could use the createProjectDescriptor method of the Settings implementation:
Settings.gradle:
def projectADir = file('../projectA')
def projectBDir = file('../projectB')
def rootProjectDescriptor = settings.rootProject
settings.createProjectDescriptor(rootProjectDescriptor, 'projectA', projectADir)
settings.createProjectDescriptor(rootProjectDescriptor, 'projectBFake', projectBDir)