I don't know DVD Studio, but maybe this will help.

If you've got video with embedded CC you can usually strip it out & convert to .srt using SCC tools & General Parser, or CCExtractor, which is based originally on scc tools. If you create CC &/or subs, IMHO use .srt to begin with. If you have scc from somewhere else, SCC Tools includes conversion tools to get it to .srt. As .srt you can use a bunch of useful 3rd party apps. UruSoft Subtitle Workshop is one of them, & will export your captions/subs to the MAC DVD Studio Pro format DVDA [& many word processors] likes just fine, though not too many 3rd party apps will work with it. [Note: after importing you need to save & reopen your DVDA project to get subs displayed without double line feeds.]

Adobe & DVDLab [don't know if or about others] can embed CC/sub text files in an mpg2 stream without re-encoding, but embedding CC in DVD mpg2 is getting rare as the few DVD/BD players that might handle CC won't pass it to the TV unless you're using SVID or lower quality connections. I think Adobe might be one of the last hold outs using scc format, never-mind that .srt is what most of the useful tools use.

Since DVD/BD discs use graphics-based subs -- DVDA creates them from those text files -- there's also several tools for working with them that way, including apps to convert text files to graphics. When/if you have graphics based sub files, &/or your DVD authoring app doesn't accept subs at all, there's still a round-a-bout way of getting them in there... Muxman will create a basic DVD [no menus] using your video/audio tracks + graphics-based sub files. VobBlanker will replace the title VOB files in your DVD with the Muxman version. Each step [Muxman & VobBlanker] takes about as long as copying your DVD files from 1 folder to another. As a 3rd step you may have to set the color of your subs, which is trivial, & can be done in a few apps very easily.

BDs are a different story, with many tools still experimental. BDSup2Sub converts graphics based subs from one size to another, say DVD to BD & the reverse. This can be handy if you're re-purposing your content, or maybe taking DVDs you've created & popping them on BD to save storage space. BD players will upsize DVDs fine, but the subs look nasty, & while you can access the audio & video on your DVDs just fine, you'd have to OCR subs to get the text DVDA needs, To use graphics based BD subs you can use TSMuxer for no-menu BDs, or try multiAVCHD for editing/modding the BD you created in DVDA -- experimental, it's worked in tests where I created a 1 word/event sub in DVDA as a placeholder to be replaced by the graphics sub file.