SQL Developer will execute any query. So you simply have to include the sum in your query.
Or what do you mean with "computed sums"
in standard SQL Plus or using other query tools one can do
break on report
compute sum of x on report
This does not seem to function in SQL Developer - but I can not tell if it is a syntactical or not. When I do HELP in the sql worksheet - I do not see "compute" listed.
We don't support break - if you want a sum by groups, you'll need a select sum(x), report from table group by report, type of query