It is possible to underfelt an old roof that was previously un-insulated by using an extended method similar to that devised for the installation of lapVent. However, 'if it ain't broke, don't fix it'!
We would suggest trunking your extraction fan outlet to a central part of the roof-space and letting the natural ventilation of your unfelted roof deal with it naturally.
Just in case though, it would be advisable to check the roof-space periodically through the seasons to see if there is condensation settling anywhere. If there is please get back to us and we will advise further.