Object Class Document with relative attachments 1:n

In our project we have a request:

• Create Object, named XXX under Document with it's custom properties (done)
• Create Object, named XXX_Attachments under Document with it's custom properties (done)
• User should be able to add, modify and remove as many documents (XXX_Attachments) opening the main document (XXX) as he want and save it. For instance. I'm open my app, clicking on XXX document, reading information and reading his main document stored on FileSystem (downloading it). Then I should be able to add the attachments.(To do)

Now, I'm thinking about, is it possibile to link XXX_Attachment with XXX in Many-To-One cardinality. For instance, the document received from external resource, saved as XXX, then an user should be able to add the attachments to it. How we can achieve that? Only via ACCE (Content Process Engine - console) or only by developing through the API?

Cheers

After couple of months of investigation, we've discovered that the thing is managed by custom plugin. So we've found the code added our portion and that did the trick.