Can I log PIC time while flying solo in an R22 toward a helicopter addon?
Solo time is always PIC time. You're alone in the aircraft, so who else could be PIC?

14 CFR 61.51(e)(ii):


  (e) Logging pilot-in-command flight time. (1) A sport, recreational,
  private, commercial, or airline transport pilot may log pilot in
  command flight time for flights-
  
  [...]
  
  (ii) When the pilot is the sole occupant in the aircraft;