It depends, as always, upon your requirement. Alfresco is not an XML database, although at one point we were considering the merits of adding an XML database.
If you have a simple requirement then you can extract the content of a few XML fields as alfresco properties. There are various XML metadata extractors, or you can write your own. Or you can use a text search of an XML document. May not be the best but could work for some requirements. Or you could somehow add an aspect with your metadata.
If you need to model some of XML's complex structure then you can, but I suggest you think carefully about how much is needed.