I'm not sure that's possible, as far as I can tell : 

Outsystems allows you use 2 different types of functions as calculated fields in an aggregate :

1) functions that are offered by the dbms, Outsystems can parse them into the to be executed sql, and so they can be executed by the dbms on the actual data, so you can refer to database fields in there

2) functions that are not offered by the dbms, so Outsystems executes these before executing the actual query, and uses the result of these as parameters into the sql to be executed.  Here you can use any server side function you like, including the ones you created yourself, but as they are executed before going to the dbms, you can't refer to values of individual rows. But all this really is, is a more compact way of calculating some stuff beforehand, and than using that as variable in your aggregate.  This is very compact and uncluttered and makes some level of preprocessing available to client side aggregates without needing some flow logic before it (i.e. without being forced to put it into a data action)

Your example above is of type 2

Thanks for your reply @Dorine Boudry