You can export it to an IDML file - but that's about it.
Better for the original creator of the file to do this.
But do .idml export kill assigment links?
To open .idml means to open "untitled" doc, so assignment are broken, I guess...
I'm not entirely sure - I don't use assignments regularly.
I wouldn't think they would be broken, they are part of the file-make up after all, the only reason I can see for them to break would be if the plug-in was associated with the assignments, then of course that functionality wouldn't be carried over - thus it might break the assignment.
You might be able to see what plug-in it was by opening the original file and going to Help and CTRL/CMD click on the About InDesign.
This should give you a lot of information about the document - not entirely sure if missing plugins would show up?