You can do this by editing your System Profile settings, going to 'Measures' and selecting 'Create Measure'.
Once here, search for 'Web Request Response Time' and that should give you what you are looking for. You should have to specify the URI and/or Query for it.
Additionally, if you already see the particular Web Request you are interested in on the 'Web Requests' dashlet, you should be able to right click on it, and then select 'Create Measure' and that will work as well.
Hope this helps,
There is a OOTB measure named " PurePath Duration". You can include this in the BT's Calculation Criteria and see if this gives you duration of the PurePath instead of PurePath response
Also interested to know about the use case you are trying to create with this. Let us know