I came across this question after searching for a ODBC or JDBC. To my surprise, since I am new to db4o I found there are tools to browse db4o, including a Netbeans and Eclipse plug in. However, when it comes to the question at hand, I only found one company, and the product is not being sold nor demoed (makes me think is not ready yet).
So, how do you transfer data? Is there a tool or script I have not found yet?
Data transfer between db4o and relational Databases (in both direction) can be easily done with db4o's tool: dRS - db4o Replication System (based on Hibernate). BTW, db4o user forum might be very useful, you will get answers directly from core developers.