Adding an additional Private Pilot category requires the full 40 hours of training. It is “almost” like starting from scratch.
Example: You are a private pilot airplane and want to add private pilot weight-shift control (WSC) trike (Note: adding sport pilot trike is much easier with a proficiency check) . Adding a trike category at the private pilot level is the same as adding a helicopter, gyroplane, balloon or Powered Parachute rating per 61.109.
There is some relief going from airplane to trike, trike to airplane, airplane to PPC, etc. private pilot because you do not have to take a FAA knowledge test, and 10 of the 40 hours training reguired can be in an airplane or the category you already hold. However, you must be trained buy a CFI or CFIS with a private pilot WSC Trike rating and take a checkride with a FAA DPE. The specifics are in FAR Title 14 Part 61.109.
If you want no hours requirement to add an trike or PPC rating at the sport pilot level, you can be trained by one CFI and take a proficiency check with another CFI for you signoff.
The same is true for starting with a helicopter, trike, PPC, etc certificate and going to airplane. Adding the the private pilot category requires 40 hours of training and adding a sport pilot category there is no minimum hours requirement with a proficiency check.