To add to s.t.; to add numbers (this uses the mag- prefix and with the mag- prefix it can also be a noun meaning addition).
[The -an imp. focuses on the object that is having s.t. added to it, whereas the pa--un imp. focuses on the object being added to s.t. The pagsi-un imp. focuses on adding two or more things together.]
If your coffee isn't sweet enough, add half a spoon of sugar to it.