Adrenaline is now open source (XMB Vita Emulation)

[LunaMoo]

Code:

- What is Adrenaline? -
  Adrenaline is a software that modifies the official PSP Emulator using taiHEN CFW framework
  to make it run a PSP 6.61 custom firmware. Thanks to the power of taiHEN, Adrenaline can
  inject custom code into the IPL which allows unsigned code to run at boottime.

Which pretty clearly explains that PSVita has a low level emulator running actual firmware and that adrenaline modifies it to run different firmware.
PPSSPP on the other hand is high level and from the very design doesn't deal with any firmware at all and never will be.
If you want a low level PSP emulator that could run your favourite firmware with it's outdated XMB ui, better create a completely new emu from zero. And if all you want is XMB looks, go hunt for some frontends/launchers which allow enough customization to simulate whatever looks you want.