Adding keywords

I want to add keywords to web pages and articles I have added/downloaded into Endnote.

Is there a way I can add them?

you mean other than one-by-one?  If you have a group of records you want to add the same set of keywords too, it is the tools Change/Move/Copy option under tools (in the Windows version).  Make sure you have just the records SHOWING that you want to change. (it will attempt to modify all the showing records, not just those selected).  This is not easily reversed, so backing up the library isn’t a bad either either.  Read the pop-up as it will warn you how many records are about to be modified.  

Tools> Change/Move/Copy and there you can add to or replace the contents of the specific fields (and include a carriage return before the item, if you want the keyword to be on a new line).