Is there a way to see if I had previously programmed
a key by Code or by Learning
If you want to find out which buttons you have learned on, put the 3000 in learn mode. Learned buttons will not flash. If you want to change the preset code you will have to delete all learned commands first.
Can a button programmed by Code by added
to with some extra learning?
Nope. But what did you have in mind and maybe there is another answer.