Can trim timber/mouldings be added to the surface of plain blank fire doors? 

For certainty on this matter, always contact the door leaf / door blank manufacturer for advice.   There isn't much in the way of information on-line on this particular subject but this is quite helpful:  https://www.falcon-timber.com/downloads/

Typically guidance on applying facings might look something like this: 

Technically adding a decretive moulding to the door should not be a problem.  You need the global assessment for the door so that you use the correct moulding in the correct position and adhesive.