I don't know of any "button" for that, but something worth trying.
My friend recently sent me her thesis and when I opened it, it said something like "this file's citations were created by the Mendeley account of x. Do you want to connect/or something with this account etc etc.".
Maybe something wort trying is to open a new Mendeley account, logging into that in Mendeley desktop and then opening your file from the other account. I wonder if you can "capture" all the citations from the file like that.
Other than that, I don't know of any miracle button ;)
Podge