There is not a way to increase the score using a content filter rule, but there are other options.
1. You can write a content filter rule to rewrite the subject to label it as spam and move it to the spam folder. It won't have the spam score in the subject though. To move the message to the user's spam folder you'll need to use macros to create the path to the user's mailbox. Here is an example rule:
[Rule004]
RuleName=Mark as spam
Enable=Yes
ThisRuleCondition=Any
ProcessQueue=BOTH
Condition01=FROM|contains|AND|domain.bad|
Action01=header search and replace|"Subject","(.*)","*** SPAM *** $0","0,1"
Action02=copy to dir|"C:\MDaemon\Users\$RECIPIENTDOMAIN$\$RECIPIENTMAILBOX$\Spam.IMAP\"
Action03=delete|
Rewriting the subject can be very complex depending on how the subject is encoded, and my action may need to be tweaked. Personally, if I didn't need to rewrite the subject, I'd just remove that part.
If you want to increase the spam score, you'll need to write your own customer spamassassin rules. This is significantly more complex, but entirely doable. There are several examples in the C:\MDaemon\SpamAssassin\rules\80_MDaemon_scores.cf that show how to check for the existence of headers and check the value of headers.
If you are just wanting to add/remove from the spam score based on the sender domain, you can use the allow/block lists in MDaemon under Security / Spam Filter. By default a match to the allow list subtracts 100 points and a match to the block list adds 100 points.