Is is possible to pull a transaction attribute into the parser XSL?

Summary:

I have a need to only update certain attributes when they are blank. The Import integration works great, except it updates attributes every time it is called.

I would like to use the value of an existing attribute in an if in the parser XSL so that it only updates when one of the attributes is blank/empty string.

Is that even possible with the parser XSL?

Below is what I tried with the customer number attribute, but it doesn't seem to pull the custNo value from the transaction.

Thank you for any pointers.

<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"