It is exactly that conundrum that got me into sleeving and more radical mods than a nip or two on a case. You can simply get 4 pin extenders like this Akasa PWM Extension Fan Cable http://www.frozencpu.com/products/11779/cab-371/Akasa_PWM_Extension_Fan_Cable_-_300mm_AK-CBFA01-30.html?tl=g47c251s637
(I sometimes do that).
Or, yes, you can solder an extension to the fan wire although doing that does void any warranty the fan has. I do that with 18ga stranded wire (and then sleeve it). Because of the short distance there should be no appreciable dropoff either method used
EDIT: Realized that extension was fairly short - here's a much longer option: ModRight Black-Out Series 4-Pin PWM Extension Cable - 48" http://www.frozencpu.com/products/16437/cab-953/ModRight_Black-Out_Series_4-Pin_PWM_Extension_Cable_-_48.html?tl=g47c251s637