Do you need 40 hours in a helicopter to get a PPL add-on if you already have an ASEL PPL? FAR 61.109(c) says: 


  For a helicopter rating. Except as provided in paragraph (k) of this section, a person who applies for a private pilot certificate with rotorcraft category and helicopter class rating must log at least 40 hours of flight time that includes at least 20 hours of flight training from an authorized instructor and 10 hours of solo flight training in the areas of operation listed in §61.107(b)(3) of this part, and the training must include at least...


The question is, does the 40 hours have to be in a helicopter, or can hours in ASEL meet this requirement? All the training centers seem to be saying it has to be 40 hours in helicopter. Can you theoretically fulfill the other requirements inside the 20 hours training and 10 hours solo in heli and be compliant with 61.109(c) with only 30 hours in heli?

Actually, it's not clear to me if the 20 hours of dual has to be in helicopter.
Considering the difference between legal and smart, you could nonetheless potentially satisfy the training requirements with just a few hours in a helicopter. FAR 61.109 specifically notes that certain requirements must be satisfied in a helicopter:


  
  61.109(c)(1): "3 hours of cross-country flight training in a helicopter;"
  61.109(c)(2): "[...] 3 hours of night flight training in a helicopter that includes" a 50nm flight and 10 takeoffs and
  landings
  61.109(c)(3): "3 hours of flight training with an authorized instructor in a helicopter in preparation for the practical test..."
  61.109(c)(4): "10 hours of solo flight time in a helicopter..."
  


Theoretically, the flight training could be all combined, resulting in 3 dual and 10 solo hours in a helicopter, the rest of the requirements being fulfilled in a different category or class of aircraft.

The FAA issued an interpretation (O'Mara, 2012) about these rules in 2012. The discussion was about weight-shift-control aircraft (61.109(j)), but the reasoning is applicable to other sections of 61.109:


  In your letter you indicate that an examiner you spoke with stated that all of the aeronautical experience requirements specified in § 61.109(j), to include the 40 hours of flight time, 20 hours of flight training, and 10 hours of solo flight training must be completed in a weight-shift-control aircraft. That statement was incorrect.
  
  [...] 
  
  ...[T]he requirement for an applicant for a weight-shift-control aircraft to have 40 hours of flight time may be met by obtaining flight time in any category and class of aircraft.
  
  Paragraphs (j)(1) through (j)(5) of § 61.109 specify the flight training that must be received by an applicant to receive a weight-shift-control aircraft rating. Flight training is defined in § 61.1 as "training, other than ground training, received from an authorized instructor in flight in an aircraft." Similar to the definition of flight time, it also does not include a limitation that the training be obtained in any particular category and class of aircraft.


That said, this is somewhat academic: in order to get a solo signoff from a CFI, a student would at least require nontrivial training time in a helicopter. I'll also cite another interpretation (Zomnir, 2010) I came across in my research, which has perhaps the most obvious statement in an FAA legal interpretation:


  First, you ask whether your student is required to be proficient to the practical test standards to pass a practical exam. The answer is yes.