We are hiring a third party administrator to pass all eligibility to our HMOs and pay our premiums with a charge back to us. Can we add the monthly per employee fee from this administrator to the HMO premiums (and then load 102%) for COBRA. Does anyone have any experience with this?

No. You cannot add any fees onto the premium charged by your carrier. COBRA only allows you to charge the insured the base premium plus a 2% administrative fee. No other fee may be attached.