Hi Alex,
Unfortunately, there isn't a custom field as what you're seeking, Owner field of portfolio items/user stories is a Rally built-in field.
I can certainly understand how an improvement of this functionality is beneficial and would like to encourage you to promote this idea in Rally Ideas (https://ideas.rallydev.com) and provide any discussion items that you think might be valuable.I can find one of them is similar with your one. You can vote this idea as well.
https://ideas.rallydev.com/ideas/D4365
Kun