Blu Ray Subtitling

Does anyone here have any experience with creating subtitles for Blu ray

We have the subtitles created for the DVD project and need to "port" them over to the Blu ray version.

One wrinkle is that we have subs moving around the screen (in some instances so that the onscreen graphics aren't obscured we have moved them to top of the screen).

So should we just be able to give the authoring house the STL file as created (externally) for the Blu ray (unfortunately we aren't authoring it ourselves).

Will all the commands which pertain to the on screen position be honoured or is there some software we can get that will export the STL file as the appropriate BDN XML file for use in Blu Ray with the associated graphics files?

Thanks