The only way you could overwrite an existing baseline is either by re-baselining the whole plan or selectively changing the baseline on those existing tasks.
A baseline is nothing more than a snapshot in time of data for those tasks that are selected for baselining. If you add new tasks to a set of tasks that are already baselined, select the new tasks and set the baseline for those tasks, it will have no impact on the other baselined task data.
Keep in mind that Project has the capability to save up to 11 separate baselines, the basic baseline and 10 other numbered baselines. So let's say you have 10 phases in your plan, you could set a separate baseline for each of the 10 phases. However, if you are using earned value metrics, you can only select one baseline at a time for the earned value calculations.
Hope this helps.