Pretty sure they can – The Book of Darkest Sorrow was written by that witch AND Declan sems to create new wasy of using magic. Spells seem to be like a simple way of automatically carrying out an action, with the same effect each time.
What Declan does seems to be more like magic manipulation on the fly, whilst the other witches rely on structured spells they can create or mold. As they explain, many use different methods – runs, magic sand etc.
There seem to be basic “rules” you can’t easily avoid – circles, channeling of power etc, but the effects change.