XML : Accessing xpath queries from property files

How do I load xpath queries from a property file ?

when I tried doing this, it simply printed the text equivalent of the xpath query, rather than evaluating it.

props.xml file:

  <strings>  <string id='msg012'>/soapNs:Envelope/soapNs:Body/schNs:processArchiveRequest/schNs:fulfillmentRequest/schNs:requestHeader/schNs:requestID</string>    

xsl file:

       <sample><xsl:value-of    select="document('props.xml')//string[@id='msg012']"/></sample>    

This post ( Read properties file or a text file in xsl) talks about substituting a text value from a property file. but, I am looking for something that loads and evaluates an xpath query from a prop file.