One other thing you didn't know about XSLT is going to mess up that plan, no matter how you decide to store the data: once you assign a value to an xsl:variable, you subsequently can't change it. So you'll need to go back to the drawing board and design a completely different solution to your original problem.
Perhaps it would be better to just post your original problem? If you have a solution to a problem but don't know how to implement it at all, it's generally better to ask about the problem and not about the solution.
Edit: for example, if your problem is actually
I would like a collection that holds all the /order/method/description values
then just write an XPath expression which returns that directly. Like "//order/method/description".