Unfortunately, right now it is not possible to create a sum of other aggregate indicators with in DHIS2 but you can create a sum of data elements. May be you can provide more details on your use case and we see how to guide you.
When a data element is created and assigned to a CatCombo, it will automatically have Totals in the aggregate form as seen when we Customise (Design Data Entry Form) the dataset, this is also seen in the analytic tools where you can analyse based on Totals or Details. See image below
The Totals:3 in the image represents the automatic summation based on CatCombos while the Indicators:3 represents the indicators that have been manually created and assigned to the dataset.