A very strong +++++1 to what Dominik and Joshua said.
It sounds like you have multiple misunderstandings going on. A user story is not a sentence. I co-authored the Agile Atlas article on User Stories, and while a sentence might convey the "title" (aka card) of a user story -- a User Story is not a sentence. If you want to see what a User Story is, check out this article: http://agileatlas.org/gasps/article...er-stories
and this one http://www.scrumcrazy.com/A+User+St...+Checklist
> product owner insists on using User Stories to capture ALL work, rather than reducing velocity
Based on what you've said, your PO does not understand Scrum or User Stories. You might want to see about getting some scrum coaching for him/her.
Regardless of Scrum or the terminology, I certainly hope that your PO understands that when he/she brings work in mid-sprint, that the Dev Team has the right to drop some other work that is currently in the sprint. If the PO doesn't understand that, it could bring great harm and risk to your company over the long haul. Trying to fit 10 pounds into a five pound bag leads to Enron type situations, where it becomes too late and your company has jumped off a cliff without a parachute.