You can actually do this if you query Elasticsearch directly by using a script on a date_histogram aggregation which adds the call document to all the buckets from the start_time to start_time + duration. The following gist contains a Sense recreation that details what I mean: https://gist.github.com/jpountz/cebb8452648c36099cee
As for doing this in Kibana, I know it's possible to run scripts in Kibana for some actions but I'm not sure if this would be possible, maybe someone more familiar with Kibana could comment? At the very least you would probably need to have a file script for this as Kibana won't let you run inline groovy scripts.
Hope that helps