You can make widgets, but widgets doesn't alter the rules of the game. Game rules doesn't allow to selectively lob certain units and not others, it takes everything in its radius.
The only thing you could do with widgets is preventing to lob anything if some unwanted units are in its radius.
If I have to guess you have a problem splitting your units and instead got everything selected and moving together at all time.
There are various way to control units separately, ctrl+X (or double click a unit) and control groups...
