well I worked something out so I thought I'd share it here for posterity in case it helps someone else...

the situation is that I have a text file of timecodes that I want to use to create chapter marks in DVDA for scene selection menus (the text file was created in another product, fwiw; I use its auto scene detection. but wanted to do this project in DVDA); my video is an already encoded mpeg, again in another product

the steps I used to create the chapter marks in dvda:
1. download and install the following vegas script: 'DVDA chapter from Vegas marker' on http://www.sundancemediagroup.com/help/kb/kb_scripts.asp?page=2&sort=4&cat=0&sw=0&author=0

note: for vegas 5.0a and b open the script w/ a text editor and change the import SonicFoundry.Vegas; to read import Sony.Vegas (this info is from John Meyer on sundancemedia.com, but I thought it was so useful that I would repost it in context of this process)

2. open the text file of your timecodes and select all rows and copy to the buffer (e.g. ctrl c)

3. open vegas and select view/edit details and select markers in the dropdown in the edit details tab

4. click on the uppermost, leftmost cell to highlight all rows, and then edit/paste to paste in your text entries all at once

5. select tools/scripting, and run the script installed in 1. above; select the output file name to match your mpeg file name (e.g. file.mpg will be file.mpg.sfl), it should be saved into the same directory w/ your mpeg

6. open dvda and add your mpeg media to your project; 'navigate into' your media file; click the load markers icon and your chapter marks should all appear!

although this isn't a straight import, it seems to be the next best thing...I hope this helps someone else...