If you register for Etihad Guest, you automatically get an extra 5 kg allowance. So head to the website and register. Then for adding excess baggage:
Excess baggage discount
Etihad Guests can pre-pay for excess baggage via the Airport Ticketing Office (ATO), City Ticketing Office (CTO) and Call Centre (within the UAE: 02 599 0000; outside the UAE: +971 2 599 0844). The Excess Baggage Vouchers (Airline Ancillary Service) will be issued as an Electronic Miscellaneous Document (EMD) and can be purchased for 5kg, 10kg, 15kg, 20KG, 25kg, 30kg & 35kg, as per the conditions below.
1. Available for sale from the time of booking until 24 hours prior to departure
2. Only available on Etihad-operated flights
3. Offered at a 10% discounted rate from the applicable airport excess baggage rates
4. Sold on the premise of:
a. use it or lose it
b. no refund for partial use. For example: a guest has 5kg excess prepaid and only requires 3kg
5. EMD can be issued for weight concept, piece concept or a combination of both where applicable
6. Guests can be issued multiple EMDs for one segment. For example, 1st EMD for weight and 2nd EMD for piece