Generate OWL from RDF generated data

Question

I have simple relational database tables

drugs (drug_id(PK), traditional_name, category, factory_id(FK) ....)
chemical composition (Id(PK), scientific_name, ... )
drugs_compositions (Id(pk), drug_id, compo_id) -- this is many-to-many relation table, links drugs with its compositions.
drug_factory (id(pk), factory_name, country, address ....)

I have used D2RQ to generate mapping, then got Data with RDF format also by using D2RQ.

till now everything is good.

1. so can I generate OWL ontology based on RDF generated data ? if it is not possible then what is the easiest way to create ontology compatible with existing RDF data?

2. I know the traditional way is creating OWL, then add data to it, but in my case I already have data (in DB) and want to generate RDF and OWL, is there any roadmap easier than my approach above?

3. the real goal is generating entailments on this data to get new facts like alternative drugs, drug contraindications. and ease link it with other existing biomedical ontologies, please help or suggest how to achieve this goal by using any tools for this (protege, Jena Fuseki, D2RQ, or other tools).