How to joint two existing models in Gazebo 6?

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Hi all. I'm trying to simulate 3 quadrotors which are transporting one load using ropes. I've used this guide link text and made 3 independent quadrotors. And now, i want to joint each of them with a load. When i'm trying to add some joints via "Model Editor", i can add joints only on one quadrotor. I've read here link text that it's possible to joint two or more existing models in Gazebo 7+, however i need to use Gazebo 6, to simulate such kind of quadrotors (using px4 flight stack). So, is there are any other ways to joint two existing models in Gazebo 6?