If we are using the task feature and someone puts in a billable task, there is currently no way to convert that to an expense. Is this a feature you are working on?
It doesn't look like this is a feature we're working on, but we appreciate you taking the time to submit this feature request! If you know other users who would benefit from this, we'd encourage you to share it with them so they can add their support by way of an upvote at the top of the page.