It is most unfortunate that the Plesk devs have imposed a 100 address limit via a setting in the database. This seems oddly small to me.
If my guess is correct and when you perform certain Plesk updates that setting gets reverted to 100, I think we are a bit stuck. I don't think we'll find any way to stop that setting reverting, and I'm not sure that it would be sensible to have an ASL self-healing rule that fiddles with a Plesk database settings (just my personal opinion).
So, if it was me, I'd be looking for a way around the problem. For example, I have a special file in /etc/mail/spamassassin/89_custom.cf where I put all my little additions. In my case they are mainly whitelists and special rules.
This file obviously has global effect, which might not be what you want. In any case, SpamAssassin isn't the best place to blacklist something, in my opinion, because SpamAssassin is quite resource-intensive on the scale of things.
So another way around the problem might be to use SpamDyke in conjunction with HaggyBear's SpamDyke Control Panel for Plesk. This has per-domain(*) sender/recipient email address whitelist/blacklist facilities and the added benefit of reducing server load massively because the blacklisting happening with minimum CPU load and the message never gets to SpamAssassin if it is blacklisted.
Individual customers can be given access to control blacklists and whitelists for their domains and other settings via Plesk itself too. There is no practical limit to the number of blacklist/whitelist entries.
(*) We do not allow our Plesk users access to SpamDyke settings, but the option is there. I've never played with it, however. Note also that we are talking about per-domain settings. This means you, the Plesk admin, can blacklist and therefore block an address globally to all domains via SpamDyke, or your customer can blacklist and block an address on his domain only. You can't block at an individual mailbox level. i.e. email@example.com
can't have different settings to firstname.lastname@example.org
-- the settings are at a domain.tld level.