support expressions inside field definition (description)

a field expression description should be able to evaulate an expression.

In other words, the field expression must support eg. the $V{variable1}

In my case, I have a variable "v1"

So I want in my field expression the expression result of

"/data/Parties[identifiers/Primary/@Id="+$V{v1}+"]/@Name"

before the datasource gets its data from the xml datasource using xpath