Set value with XPath dynamically #2

I am running into a similar problem as

http://forums.adobe.com/message/2953296#2953296

.but instead of constructing a string from xpath expressions, I'm trying to store a value into a Location based on a concatenated xpath expression.

Here is what I'd like to do but it doesn't work:

/process_data/@Approver_index = 1

Location:

/process_data/formData_flat/xdp/datasets/data/TDCR/RoutingName_[number(/process_data/@Approver_index)]

Expression: 'Bob'

This works if you were to put the string into the Location but Workbench won't accept this as a Location.  How can I dynamically apply a template xpath expression?  Any help would be great.