I have approached this add-on service in a couple of ways.
First, I did it myself, offering a recoat service as "dustless" refinishing, which is what it is. Some floors are good candidates for recoating; some are not. The ideal candidate is a floor which is slightly scratched, and maybe "dulled-out" as well. The poor candidate is one where there is one or more areas where the finish is totally worn off the floor, leaving a grey, dirt-saturated area of the floor. Or major gouges or other floor damage that would require sanding to remedy. There are kits available from floor product companies such as Varathane and Bona-Kemi which are designed to be used by people who do not refinish floors (sanding) for a living. I had pretty good luck with these, although since they are water-based finishes, which dry very quickly, I sometimes ended up with some minor "waviness" in the dried end-result. Only in one case was my customer dissatisfied enough so that I didn't charge her for the job. I chalked it up to being a learning/training experience. I was charging $2 per sq. ft. for this service, and that was as far back as the late 90s. The only customers I got for this work were my own carpet cleaning customers, since $2 a foot is high for a recoat job.

A lot of people who were interested in recoating didn't have floors which were good candidates for this limited type of refinishing, so I was passing on a lot of work. After a while I decided to look into subcontracting out the "real" refinishing - that involving sanding the floor, then applying 3 new coats of polyurethane, and maybe stain as well. I searched on AngiesList for someone not too far away from me with consistently strong posts from his customers. The guy who sounded good was very receptive to working in this way, and at that point I decided to have him do any wood floor work that might come my way, including recoating. For full sanding and 3 coats, he charges me a wholesale rate - $1.25 per sq. ft., for which I bill my customer between $2 and $2.25. For recoat jobs, he charges me just .50 a sq. ft. I bill these jobs to my customer somewhere in the $1 to $1.50 range. He does excellent work. My customers love him and his crew, and now I am much more competitive than I was previously, and I don't have to worry anymore about possibly having a "wavy" result. He's even referred a couple of c.c. jobs to me, for which I send him a 15% referral fee.