The list in plesk are stored in mysql ..
There is a flat file of blacklist and whitelist in each email account but you can't update from flat file to mysql only just from mysql to flat file
But copying and pasting the blacklist/whitelist to flat file will work but it will not be displayed or listed in Plesk since it need to be in mysql database and the next update will wipe the list down to whatever you have in mysql so anytime a address is added in Plesk, the flat file is updated to match the mysql list.
You'll need to insert them into mysql database first then run the mchk -v which will fix the flat files for each email account.
If you do a search on spam and my name you will find a history of post of problem I have with sw-soft spam assassin .. they couldn't even fx it after thier upgrade screwed it all up .. I had backups of database but the upgrade keep corrupting somehow and I didn't have any choice but to install spam assassin fresh and told all my clients they had to redo thier blacklist / whitelist.
Bill