Adding phases without changing '% of Project'

User wants to make the additional services on a phased project as a phase on the project, but does not want the % of Project (Projects > Project Center > Structure tab) for the original phases to change.

You can do that by creating the additional phase with an Hourly contract type and setting its contract amount to zero. The % of Project for the other phases remains unaltered. Check CORE Help Center for more on creating project phases.