I have a stylesheet that is used for a number of
documents. This stylesheet is used to generate PDF
using FOP. Hence I am setting various attribute values
such as page size, margins, font-size, font-family etc
in the stylesheet. What I would like to know is that is there a way to
get the values of these paramters from another XML
file dynamically.

Get a book or the XSLT spec and look up the document() function.
You'll probably want to have a look at xsl:attribute-set too,
which was specifically designed to get bundles of attributes
representing a presentation style into the transformation
result.