How to better avoid "Error: free joint can only be used on top level" when combining two models

I want to create separate xml files for robot, object, arena and combine them together later.
Because all the objects are very similar, I decided to put all of them in one xml file. Each object are should move freely.
Something like this:
```XML

```

Then I use .attach to combine two models: modelArena.attach(modelObject)

But, this will create an outside

```
Then I will get a problem "Error: free joint can only be used on top level".

I know one solution would be to create xml file for each object individually and let the attachment frame add('freejoint').
But this is not very elegant, and it will create many redundant attributes (like