Kyle Boatright wrote:
Our 40 year old home has a mixture of insulation in the attic. 2/3 of the
attic has blown in cellulose, and the other 1/3 has fiberglass batt's that
were added during renovation work.

Can I blow in additional cellulose over the whole area. If not, why?

yes you can blow cellulose over the fiberglass.