If I'm understanding you correctly, you want to use an alternate origin only if a particular path has been requested.
(I'm assuming the actual path
**category** doesn't really exist) As long as you can translate
**category** into a rule that can be expressed as a regular expression then it can be done.
You'd create your new backend as per this doc but instead of using a condition that checks if the backend is healthy or not you'd use one that checked if the request was for a category. eg
req.url ~ "^/testbucket(/\d+/.*)$" or similar, depending on what makes up the category path structure. You can then add a host header override with the same condition attached.
Is that what you were looking to do?