Yeah, it wouldn't be a fixed gear bike anymore. Any bike with horizontal dropouts or track ends should be suitable for an internally geared hub (IGH) like the Shimano Nexus 8. There are a number of bikes that come stock with such hubs too. Just a few of the manufacturers that make bikes using these include Swobo, Trek, Bianchi, Felt, Brodie, Novara, Breezer, and Specialized.