I have this problem: my customer sent me a RDF file with its own
schema.
It doesn't matter that much, but what is this schema written in ?
RDFS / DAML+OIL / OWL ?
I absolutely need to convert that RDF schema in XML schema.
Does anybody knows if this operation is possible?
No, no-one knows if this operation is possible

Great minds have
pondered this for several years and the jury is still out. RDF can
express things that XML can't, so how to translate a truly generalised
schema is still a debatable problem.
In general though, it's a bit of a no-brainer. Unless you're actually
doing some of the weird stuff, there's not much to it.
I am puzzled as to why you should need to. When you have this XML
schema, what are you going to do with it ? It's a far more common
problem to have an RDF document and a pre-existing XML schema, and a
need to know how to map one onto the other. You already have a schema
(in something RDF-related), so why do you need another ? Presumably
there's some other tool involved that is going to have to parse the
RDF document, so what is this expecting ? I can't imagine you have a
need to convert the schema, but not the data document itself ?
Please tell us a bit more about the broader picture.