I have no idea about BO6.5 however I feel that creating 
Derived table must be similar in both XIR2 and BO6.5.

-right click any where in Designer.
-select Derived table from Popup menu.
-A window will open up. Give the name of the derived table, 
You can specify the SQL stmt here as you do for example in 
oracle(if you are using oracle DB).

Example:
select BONUS.ENAME Ename,DEPT.DEPTNO,EMP.EMPNO  from 
Bonus,dept,emp where EMP.DEPTNO=DEPT.DEPTNO

You can specify the join in SQL stmt if necessary.

However joining the Derived table indeed is similar as 
joining a normal table.

Thanks,
Amritha