It is possible!
Create custom ability based on the channelling ability, like starfall. costumize it with no damage, no effect, and no target.
then create dummy unit with breath of fire ability.
OK, finally create trigger so when the units starts the effect of that ability, it will activate a trigger.
the trigger will create dummy unit for each second, order it to cast breath of fire to a point, and add expiration time to it.
When the caster stops the effect of an ability, deactivate the trigger.