Modify subrequest header (no replies)

I am trying to use ngx_http_subrequest in my customize nginx module. I can see from the code that the subrequest share the same request header with the main request(sr->headers_in = r->headers_in). Is there a way to modify, add or delete request header for subrequest without affecting the request header of the main request?

I tried ngx_list_init(&sr->headers_in.headers and use ngx_list_push to push new header in. And it is giving me a runtime error. Can someone point me to the right direction?

Thanks