To add new slices go to your pie's 'Holdings' screen and tap on the 'Edit Pie' button:
You will enter the 'Edit Pie' mode. Now tap on the 'Add slice' button (+).
Start browsing and select the instruments you wish to add to your pie.
After confirming your selection, you will be returned to the 'Edit Pie' screen. You will need to free up some space in your pie, in order to accommodate the new slices. The sum of all slices in the pie must amount to 100%. Let's say you want your new slices to occupy 10% of your pie. This means you need to reduce the targets of other slices with 10% in total.
When you are ready, tap 'Save'. This, however, will not result in trading, as the 'Pie' edits affect only the target values of your pie's slices. Hence, in order to enforce the new allocation values, you will need to rebalance your pie.