Custom functoid reading source XML structure

Hello,

I need to create a custom functoid that is able to read not string value from source document as usualy, but specified part of XML tree. User specifies some node from input schema and functoid should read this node and 
all its subnodes and work with them as normal DOM. As an output I need to return normal string value found in this XML, so I cannot use custom inline XSLT functoid.

Do you have any idea how to develop such functoid for BizTalk 2006 R2?

Thanks very much,
Petr Janda