If my contributions to my company 401K plan are only $4500 ( 15% of my p/t salary), can I make up the differen?

I want to put more into a retirement plan as I am over 50. How can I contribute more to reach the govt limits. Am I allowed to put it into an existing IRA.. If so, how much cn I put in (the difference between 4500 and 6000 or 4500 and 14000, uming I have monies available elsewhere)