Jake, external hydraulic hookups are an option on these tractors. They are called "hydraulic remotes" by most people. Your tractor can have up to two remotes and the DX40 can have three. Right now, each remote attachment point and the control lever kit is around $650 if you install them yourself. I've attached a photo of my tractor with three remotes and a log splitter hooked to it. My New Holland is similar to your D40. I also have a hydraulic cylinder on my toplink and one on my right sidelink so I can adjust the 3PH from the operator's seat. This is called "top and tilt" or abbreviated TnT.
If you have a loader, you can remove two lines going to your loader and hook your splitter into the loader's hydraulics. Then you bungee the joystick in place and use your splitter's valve to control the splitter. You'll have to get longer hoses and maybe adapt them to fit the splitter's 1/2" hoses and also buy new quick connects to fit the loader QCs.
Some tractors come with a single rear hydraulic remote as standard, but your D40 does not.