More than 39000 miles?
 

I am currently in the process of planning a RTW trip next for about 6 months starting next summer. My intended routing looks tentatively like this:

ywg-yvr-hnl-akl-syd-bkk-sin-jnb-fra-cai-vie-ath-lhr-mad-eze-gru-ccs-mia-ywg

According to the Great Cirlce mapper this is 44000 miles. Is it possible to purchase any additional miles? I would really like to make this route happen. Also any comments or advice on the routing would be appreciated.

You can always buy an additional side trip.

if you stick exactly to that itinerary, than you can't just buy additional side-trips. And there is now way to extend the 39'000 mileage limit (that's my own expirience, tried in 1999).