Do you have a test case for the kind of expression you'd like to use?
For instance, something like the following:
public void test109() throws Exception
JexlEngine jexl = new JexlEngine();
JexlContext context = new MapContext();
value = jexl.createExpression("foo.bar + 2").evaluate(context);
If you are really trying to define a variable named "foo.bar.", this is not a bug but an RFE that I'd vote against adding without a use-case.