Hi!
That's one way of linking up, but another way is using equivalent property ( https://www.wikidata.org/wiki/Property:P1628 ) and equivalent class ( https://www.wikidata.org/wiki/Property:P1709 ). See for example
It is technically possible to add values for P1628 into RDF export. However, the following questions arise:
1. Are we ready to claim these are exact equivalents? Sometimes semantic meanings differ, and some properties have class requirements - e.g. http://schema.org/illustrator expects value to be of class Person, but of course Wikidata item would not have that class. Same for the subject - it expected to be of a class Book, but won't be. This may confuse some systems. Is that ok?
2. How we deal with multiple ontologies with the same meanings? E.g. https://www.wikidata.org/wiki/Property:P21 has 4 equivalent properties. There might be more. Do we want to generate them all? Why there are two properties for the same FOAF ontology - is that right?
3. If you change P1628, that does not automatically make all items with the relevant predicate update. You need to do an extensive update process - which is currently does not exist, and for popular property may require significant resources to complete, some properties have millions of uses.
Using P1709 is even more tricky since Wikidata ontology (provided we call what we have an ontology, which may also not be acceptable to some) is rather different from traditional semantic ontologies, and we do not really enforce any of the rules with regard to classes, property domain/ranges, etc. and have frequent and numerous exceptions to those.