Novorender lets you merge all child object into the parent object. This will merge all object into one object higher in the model tree hierarchy.
Open Rules for the project or a specific folder a click "View or edit rulesets
Make a new rule
In the "rules" section, select which objects should be merged into their parent object
In result select "merge" for the action in "true"
Click save
Note: You do not merge all the objects you searched for in the rule. The objects that are affected by the rule will merge into their parent object, not necessarily merge with each others.