I could be wrong, but I don't believe that's possible through our UI. There might be some API-related options you could build, but there are complications with that as well. The problem is that while we have API's that would allow you to "override" an approval, if you override the approval, it skips all other approvals in the workflow. The only way I can think of to get around that would be to develop a solution that first adds the designated approver as an approver at the current level of the workflow and then approves the request on their behalf.
Given the complexity, I wouldn't really recommend a custom solution here. Instead, I'd suggest posting this to our Ideas portal for consideration in a future release.