If you are working with MPEG video in Womble, you cannot add this kind of subtitle without re-encoding the whole file. (Edit: Unless Womble has the ability to add text captions to the picture? - then only the subtitled sections would need to be re-encoded.)
I'd recommend using Subtitle Workshop to create your subtitles, then importing into DVD Lab to add as "soft subs". See: http://www.mediachance.com/dvdlab/helppro2/subtitles.htm