Getting visa extensions in country

Hi there

I'm from the US and I went to Zambia back in 2006. When I was there, I was given a 30 day stamp and had to leave the country before then and re-enter to get another 30 day stamp. I'm heading back to Zambia again but I'd very much prefer to stay 60 days without having to take an all day bus ride to the border to get another stamp this time around. I already have my ticket for 60 days and I'll have that info with me when I arrive so I'm wondering if/where I can get a 30 day extension right when I arrive.

I will not be spending any time in Lusaka except for arrival and departure so I'd like to get it done right away as to avoid a loooong trip back to the capital to take care of it. Can it be done at the airport or do I need to find an immigration office somewhere?

Any help on this is appreciated.