flowでなくApexで同じことがしたいという質問/回答があります。
解答はApexトリガーではレポートのエクスポートを拾うことができないので、APIを使った独自のUIを作って実装するしかないと回答されています。
AFAIK this is something which cannot be implemented reason being - A trigger only executes before or after the following types of operations:
- insert
- update
- delete
- merge
- upsert
undelete on a record.It cannot be triggered on the click of a button which doesn't perform any of the above operations.
Probably you can work thrugh reporting APIs and create your own reporting UI and implement this functionality.
https://developer.salesforce.com/forums/?id=906F000000094IEIAY