To connect or join two things together; to add s.t. onto s.t.; to extend.
The -an imp. focuses on the object that is having something connected to it. The -un imp. focuses on the two things that are to be connected together. The pa-un imp. focuses on the thing that is being connected to something else. The -an is not used when a line is broken and then the same line is connected back together.
I am going to add some to this line so it will be long enough for me to go fishing.
No one can add anything to that king's thinking because in his opinion he knows everything.