BTS 2006 and xpath question

I am new to BizTalk and only familiar with xpath.  I have a xml document that will look something like this that will come into an orchestration via several receive ports.

<Properties>

<Dictionary>

     <key> </key>

     <value> </value>

</Dictionary>

</Properties> 

the key and value nodes will repeat for all the settings (like a typical app.config).   What I need to do is either update the value node under dictionary where the key node has a value of X.  If there is no key node with a value of X then I need to create new key/value nodes with the key node value of X and the value node with a value of some variable.

I’m not sure how I would do this in a construct message shape in an orch.  I think I would just use the xpath function on the message I am creating.  Basically this message is a copy of another message.  So I have NewMessage = OriginalMessage; NewMessage(*) = OriginalMessage(*);

I then need to update (or create) the key/value nodes under Dictionary as explained above.  Is this possible?  If so, what would the xml look like?  Also I am assuming that if I used an xpath expression to update the value node where the key node = X and the key node with value of X does not exist, then it will NOT create it for me automatically.

Thanks for any tips and help!