A pie-chart is based on the full circle being 100% of the data. To add additional segments, we'll need to recalculate the entirety being the old data plus those items in the new data
But When I try I lose the previous segment and color
I have already a piechart I have to have a further segment in the chart but it has to keep the previous parent segment intact.
Click on the Pie chart, in the popped-up mini toolbar click the color edit button to launch Build Colors dialog where you can specify individual color.