My usual technique is to add these scores in using excel after I've downloaded the scores for the other students. However it is possible to change student scores in the database, although it is time consuming.
From the Hmwk editor page click in the column "edit assigned users" for a given set and then choose the user whose scores you wish to change.
On this new page you can now edit the "status" column for that student -- change it to 1 for full credit or 0 for no credit for each problem. (you can also give partial credit.) If there are many sets this can take some time, but it does change the database completely and you do not have to remember to make any compensations at the end of the semester.
I hope this helps.