Unable to extend classes declared outside drl

Description of problem:
Trying to extend POJO class in DRL file leads to build error (not a declared type). However it is very easy to bypass this by adding some metadata (like marking it event) to the original class and extending it now works. Would it be possible to allow extending even the external classes?

Version-Release number of selected component (if applicable):
BRMS 6.0.1 ER1

How reproducible:
See attached drl

Steps to Reproduce:
1. build the attached drl

Actual results:
Message [id=1, level=ERROR, path=rules.drl, line=6, column=0 text=Cannot extend supertype 'java.util.Date' (not a declared type)]

Expected results:
Rules are compiled without error