Is your requirement the following?:
I export a path (test case) #ITEM-A# in ARD to a reruirement/user story #ITEM-B# in tfs.
this creates a test case on the tfs server #ITEM-C#
An external link is created in path explorer external links between #ITEM-A# and #ITEM-B#
This does not exist in the product today.
Today you can export test cases to requirements but ITEMA is linked to ITEMC
We can manually add link paths to work items on tfs. As a workaround, for each path you wish to link, path explorer ,click external links, click the + , choose tfs and link to your work item. This is cumbersome for many test cases..
Quite often a better solution applies when 1 flow models 1 (and only 1) requirement.
Click manage, properties, application links, browse to your requirement and link it.