I think you cannot do this with CMP. What you can do is that you create two different CMP-EJBs using the classes but specify two Entity-Beans using two different datasources. Now you can use a SLSB and copy the data from one EB to the other.
Have fun - Andy
Note that this will not give you correct transactional semantics unless you have an xa capable driver and you are using its xa capabilities. (i.e, you are using the real XADataSource class rather than jboss's fake XADataSourceImpl in XADataSourceLoader configuration).