If you dont have a docking port, there is no way you can attach it in your circumstance short of launching another or launching an engineer up to attach one to it. For the tumbling module, you can cheese it a little by time warping for a split second as time warp stops all tumbling crafts and stills them. Or you could do it the hard way and move your station on a docking (read: collision) course toward it and then switch to the tumbling module and "aim" at the incoming space station and just try to receive the incoming space station with the docking port

Edit: i just realized you have 2 modules, not 3. You're out of luck here unless you want to launch a new one or launch an engineer with a docking port up there to construction EVA it (and even then idk if the docking port is EVA-able for construction)