Any internist can become a hospitalist. I think that groups may be more willing to hire you if they know that you are right out of residency though, as categorical IM residencies train physicians well to be hospitalists and in-pt management skills, like all skills, atrophy with disuse. I was actually just talking with an IM resident who is graduating late who told me that she had 4 job offers already with minimal effort, a few of them were hospitalists job offers. There's definitely a demand out there. There are some hospitalist fellowship programs starting to pop up around the country, and they might make you more "employable" or train you better to do your job, but it's entirely unneccessary to do a fellowship at this point. Job opportunities are currently plentiful in the hospitalist industry. Some say that one of the reason is that burn out is common with hospitalist, and you have to keep really up to date with literature, so that's why most of them are young. To the best of my knowledge, I don't think that the American board of internal medicine recognizes hospitalist fellowship/board certification either.