Listen up kiddies, school is in session.
To add a low pressure auxiliary injection system to your OEM direct injection system requires building a complete, secondary fuel system. You cannot piggyback off of the low pressure side of the stock system because the pressure fluctuations are too great and it would be impossible to tune for by just adding an extra line.
Parts required for the install include an intake manifold suitable for port injection, aux rail/injectors, pressure regulator, fuel supply and return lines, a fuel filter , tach adapter and a controller for the extra injectors. The new pump has to pull from its own supply so you can either use the stock lpfp to feed a surge tank and have two pumps pull from that, each to feed the stock and new aux system or convert to an R32 tank with transfer pump as a feed. The surge tank option is massively easier to pull off.
I learned the hard way that the AEM and 034 aux injection controllers will not work with a DI system to pull signal from the stock injectors to run the auxiliaries because they use a 60v signal instead of a conventional 12v. Split Second makes a controller that uses ignition signal to control the aux injectors but it also requires a tach adapter to convert the signal for the controller. Autometer makes a good unit for that.
I make adapters for the stock LPFP to go to AN -6 fittings so that you can run lines to a surge tank. Don't think anyone else makes these, I developed them for myself and can make some for others if they needed. As far as I know, no one else is running AN fittings off the stock LPFP feed fittings.
