There is an application called PrimaveraReader, although it is not free.
It is about EUR 250 per user.
The PrimaveraReader can be used to open a XER file and even update activities via XLS.
See the link below for some information:
Oracle has the XER parser that you can download from the support site. You need to have a valid support account with Oracle.
This is relevant KB article: https://support.oracle.com/epmos/faces/DocContentDisplay?id=902053.1