Yeah, this is one of Construct's biggest limitations imo - the inability to build prefabs beyond containers. Workflows in Flash, Godot, Unity and Unreal lean heavily on this concept. I'm desperately hoping Scirra will implement a scene graph once the C3 runtime is up to snuff. The suggestion for that is already logged and has a ton of support from the community, and it sounds like they plan to look into it, but a few more votes wouldn't hurt... <img src="{SMILIES_PATH}/icon_e_wink.gif" alt=";)" title="Wink">
https://construct3.ideas.aha.io/ideas/C3-I-12
In the meantime I have been looking into various ways to deal with this problem in my own project, which requires quite a few composite objects of the type you're describing. The suggestion to hard-code each one is the simplest and if you're fairly new to Construct or if you don't have many of these object groups that's what I'd suggest too.
Another (considerably more complicated) method to consider if you want to push things further would be to build an object editor for your project. Basically you'd create a level editor, but for objects. The short and incomplete version is you'll loop through the objects in your group, then save the data for each object into an array or dictionary using a combination of the objects' names and the asJSON system expression. Then you'll write the array or dictionary to a file (you will need to export to NWjs for this). Then at runtime you can recreate the object(s) from the file(s) using AJAX to load the file, create object by name to grab each object name from your array, and loadfromJSON to set all their properties like position, etc. Finally you pin them together, if that's needed - this also requires you to track some kind of pin target uid when you create the objects.
I would suggest using families for all this to make such things easier - for example, I'm using a "Serializable" family with an "ObjectType" variable to store the object names, and a "Pinnable" family with a Pin behavior for all the things that need to be pinned. This way you can use functions to simply get "Serializable.ObjectType" for every single object in your group, and loop through them more easily. I'm currently hard-coding what things in a group get pinned to based on their object types, but there is probably some good way to abstract this further with yet another family ("PinTarget" or something).
Anyway, it's a little complicated and difficult to explain fully in a short paragraph like this, and it's probably overkill for your problem, but that's the basic idea.