DirecTV slimline setup question

You don't need to do that. If there is only 1 output then it IS a SWM antenna. You can add a 2 or 4 port SWM splitter (pictured above) to the output and then run your coax from that connection. The PI does not need to be between the antenna and the splitter so you could locate the splitter at the antenna, run the neighbors coax from the port 1 connection (the only 1 with a DC pass) and run the coax to your receiver from any of the other ports. You will not need to install a power inserter at your receiver but you will need to set the receiver to SWM.