Well, it depends on how you define "keywords". If your definition is how it is defined in the specification -- meaning reserved words to the Java language, and can't be used as identifiers in your program -- then no, you can't define new keywords.
You could also use a tool like the Java Syntactic Extender, but it's a *quite* old project (I first used it back in, say, 2003? 2004?) and may not support recent Java source code. It was, however, quite interesting, and (rightfully so) a source of endless consternation for my boss at the time.
So basically the answer is no, because Java is not a language that lends itself to doing such things.
Note that some DSL techniques can sorta-kinda make it look like you're adding keywords, but the syntactic verbosity of Java generally eliminates any usefulness of such trickery.