Should not be too difficult if the bike is steel as the rear dropouts can be widened as necessary for the new hub. If the frame is aluminum this cannot be done. Different hubs have different widths from about 120mm to 135mm.
Any good LBS can build an IGH wheel or you can get the parts and learn to do it yourself. Harris Cyclery does a lot of IGH wheels per their web site and have some ready built wheels.
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Gear Hubs Owned: Rohloff disc brake, SRAM iM9 disc brake, SRAM P5 freewheel, Sachs Torpedo 3 speed freewheel, NuVinci CVT, Shimano Alfine SG S-501, Sturmey Archer S5-2 Alloy. Other: 83 Colnago Super Record, Univega Via De Oro
Visit and join the Yahoo Geared Hub Bikes group for support and links.
http://groups.yahoo.com/group/Geared_hub_bikes/