Hi there,
The answer to your question is, yes you can complete an allopathic fellowship after a surgery residency. The key to getting any fellowship is to do very well in your residency both academically and technically. Work hard, do some good solid research and get to national meetings to present your research. By going to the national meetings, you get to make contact with leaders and others. Getting your research published is also a good thing. One of my DO colleagues finishing a DO residency, who is an ace surgeon, has been deluged with invitations to interview for fellowship. He meets the above criteria. Work hard and study even harder.
this comes up about once a week and it seems we never come to a consensus. A few of us are under the impression that you can do the fellowship but will not be board certified by the allopathic folks. Thus, you will be fellowship trained but not board eligible. I've only heard about this in passing so take it for what it's worth.