Both SRT and SUB are fairly common movie subtitle formats, you can easily convert them in all subtitle editors and creators. Just pick one of the programs listed below and save your .srt files as .sub files.
Articles and other information relating to srt into sub conversion:
Create subtitles for 3D movies - How to convert 2D subtitles to 3D subtitle file format with 3D Subtitler and Subtitle3d.com tools
Best free software for movie subtitle editing - Top free software for creating and modifying of movie subtitles