On Palm OS, not yet, though it almost certainly will be in 2.0. On Pocket PC, if you quit PlecoDict (using the Quit command in the Dict menu) and move the flashcard and user dictionary database files to the card in File Explorer, it should be able to access them from there when you start it again.
I got it to work on a PalmOS based system (Treo 650) but there are drawbacks. You didn't mention whether you are using a PalmOS based PDA or a Windows based PDA.
I got it to work using a third party launching program. Refer to the following thread for further information about the drawbacks of transferring the flashcard/user databases onto an SD memory card: viewtopic.php?t=643
I'm not sure if something similar will with with a Windows based PDA.
If you require further information after reading the above thread, I can post detailed information on what I did and what launching program I'm using.
However, since this is not really supported, you might run into problems that I didn't run into even using the exact same programs/configurations that I'm using (although I doubt it based on how the launching program works).