Ok, I went to the West Marine store and talked with their expert. He suggested that I use the "West System" 105 Epoxy Resin with the 205 Hardener as a filler.
To prepare the Balsa core fiberglass panel -I drill through the upper layer of fiberglass only. Then, using an allen wrench attahced to a drill motor, I waller (southern word) out the balsa wood core larger than required. I then pore in the mixed Epoxy Resin/Hardener and let cure ~ four hours +.
Then, I drill through the filler and the opposite side to install my marine component with a bolt, washers and nut. This way the balsa core is unlikely to be exposed to a water leak or other.
Balsa core fiberglass structures are used widely in boat to reduce cost and weight. I'll let everybody know how it works out.