OK, I've found a partial answer my own question...
First, you have to unzip your Stardict dictionaries so that you have three files (the dictionary name plus the extensions *.dict, *.idx and *.ifo). N.B. You can download dozens of Stardict dictionaries here:
http://yeelou.com/huzheng/stardict-dic/.
Use a program called CvtStarDict2Txt (Google to find free download) to convert the dictionary to TXT format.
In most cases, the resulting TXT file is already formatted as (word)<tab>(definition) so it can be directly run through Pleco's "MakePlecoDict" program (available on this website). The results will be rough but usable. Editing the TXT in Wordpad, MS Word, etc. can help clean it up. For example, I find all of the "/n" line break characters & replace with empty spaces. If anyone has a guide, macro, etc. for formatting the TXT dictionaries to better suit Pleco I'd love to see it. Changing the TXT format to (word)<tab>(pinyin)<tab>(definition) would be great for Chinese-to-English dictionaries.