To solve this easily we need to get the addition flag field in script like below
If(IsNum(AccountNum), 0, 1) AS NonDigitAccount
Now in expression you can directly use the below expression
If you don't want to change the script then you can try below expression
=Sum(Aggr(If(Not IsNum(AccountNum), MeasureName), AccountNum)) // The performance of this expression is not better when compared to the previous one.
** Note: Replace AccountNum and MeasureName with your actual field names.
Hope this helps you.