Edit a Format or create custom Formats

[godirk]

I need to frequently convert subtitles from different formats, often .stl to .dfxp. The Netflix Timed Text format almost meets all the specs I need, but any positioning other than top and bottom center that may have been there before is lost when I select it. I guess they only allow those two. Would it be possible to create a new format that does all the things the conversion to Netflix Timed Text standard does but without loosing all the positioning information that is not top or bottom center?