Apparently, after a long search, I found out that there is no easy solution to combine an Rpi with a UPS Hat and the Amp+, since both the UPS Hat and Amp+ demand their own power supply and also feed the Rpi.
Since the Amp+ is something I want, and the UPS Hat is something I need, I was wondering if it is possible to provide the Amp+ with it's own power supply while only connecting/attaching the pins 18 through 21 to the Rpi. (The Amp+ would not be fitted onto the Rpi but would lay next to it.)
The UPS Hat can then control and provide power to the Rpi. See schematic below.
Is this possible?