You can extend the module (besides using the yum::repository definition), by adding profiles other then yum::standard, yum::testing, yum::development, yum::eatbabies, for example, yum::myprofile. To do so, create a class in classes/yum.pp as follows;

class yum::myprofile inherits yum::standard {
    yum::repository { "custom1":
        enable => true
    }

    class disabled inherits yum::myprofile {
        Yum::Repository["custom1"] {
            enable => false
        }
    }
}

To have a node be configured to use the custom1 repository (enabled), use:

node 'node1.example.org' {
    include yum::myprofile
}

To have a node be configured to use the custom1 repository (disabled), use:

node 'node1.example.org' {
    include yum::myprofile::disabled
}

Additional customizations, such as a myprofile2 with additional custom repositories, plugin configuration or update schedules, should go into classes/yum.pp as well.