I know that it is possible to separate a complex hatch object (that was defined by multiple boundaries and objects
in a single hatch command session) with the Hatchedit command and the setting for "separate hatches" activated.
What if we have created some hatch objects (in a single hatch command session by clicking into various boundaries)
with the setting to "create separate hatches" activated by mistake? Is it in some way possible to stick all the
separate hatches back together in a single hatch object (without deleting them & placing hatch from scratch)?

creating a block as you suppose would only be a temporary & rather not suggested solution as it would add unecessary block definitions, and the resulting object would not be editable with Hatchedit (as it wouldn't be a Hatch any more) Moreover if you exploded the block-hatch it would return directly to previous status.(leaving the definition behind / purge etc).If we were to stick hatches together in this way,I would suggest creating groups that are by far more flexible than blocks in these types of actions.