Adding an additional Private Pilot category requires almost 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 an FAA knowledge test per 61.63 (b), and 10 of the 40 hours training required 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 an FAA DPE. The specifics are in FAR Title 14 Part 61.109. See WSC Trike Private for more details on initial or adding a WSC Private certificate.
If you want no hours minimum requirement, just the hours to gain the skills, to add a 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 private pilot category requires 40 hours of training per 61.109 and adding a sport pilot category there is no minimum hours requirement with a proficiency check.