Can I install an 8 speed sturmey archer hub on a 6 speed dahon folding bike ?
I'm not sure whether there are any challenges specific to a dahon bike, but here's a summary of the challenges you'll face converting any derailleur style bike to an internal gear hub:
Other than this there shouldn't be any reason you can't do this. Note that an internal gear hub will be heavier than your existing derailleur setup.
Given Dahon make other bikes with internal gear hubs it might be worth considering purchasing one with all the gear already sorted.