The solution was achieved by upgrading to version 2.1 ejb, because there the "version" attribute is allowed and this is required by WebSphere to recognize a non-EJB 3.0 version.
This means that an EJB 2.0 version can not work, since the above tag is not allowed in the ejb-jar_2_0.dtd. Maybe a
<cmp-version>2.x</cmp-version>
could help here, but I didn't tested it.
The conversion of the header of ejbjar.xml brought success:
from 2.0 (ejb-jar_2_0.dtd):
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE ejb-jar PUBLIC "-//Sun Microsystems, Inc.//DTD Enterprise JavaBeans 2.0//EN"
"http://java.sun.com/dtd/ejb-jar_2_0.dtd">
<!-- EJB-jar file declaration -->
<ejb-jar id="EJBJar" version="2.0">
<display-name>Overall Bean Definition</display-name>
<enterprise-beans>
<entity id="Dcnotetext">
...
to 2.1 (ejb-jar_2_1.xsd - you need namespaces! ):
<?xml version="1.0" encoding="UTF-8"?>
<ejb-jar id="EJBJar"
xmlns="http://java.sun.com/xml/ns/j2ee"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
version="2.1"
xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/ejb-jar_2_1.xsd">
<!-- EJB-jar file declaration -->
<display-name>Overall Bean Definition</display-name>
<enterprise-beans>
<entity id="Dcnotetext">
...
No further changes to XMI or XML files were necessary!
Thanks for help!