This is the only thing I could dig up on the subject. Most other specific rulings which I found seem to apply to people working for the parish, in schools etc.
This is all in German, but
Google Translate is your friend.
Basically is seems that as long as it is not taking place in your employer's time, is not for a 'competitor' of your employer, does not affect your ability to carry out your usual job (by making you too tired etc). then it is permitted.
Don't forget your tax forms and maybe a check on the need for extra insurance though, will you?