From the legal perspective, I would suggest "no".
Here is the JAR FCL reference that suggests it is not legal. I'm not up with EASA references but there's probably an equivalent somewhere.
JAR–FCL 1.010 Basic authority to act as a flight crew member
[(See Appendix 1 to JAR- FCL 1.010)
(See Appendix 2 to JAR-FCL 1.010)
(See AMC No. 1 to JAR-FCL 1.010)]
(a) Licence and rating
(1) A person shall not act as a flight crew member of a civil aeroplane registered in a JAA Member State unless that person holds a valid licence and rating complying with the requirements of JAR–FCL and appropriate to the duties being performed, or an authorisation as set out in JAR–FCL 1.085 and/or 1.230...