By impersonate if you mean you are wanting to basically log in as another existing portal user - there is no out of the box way to do it. If your users are tied to an Account, and access is at the account level (i.e. not scoped to the individual Contact), then you can just add your Contact record to the same Account.
If you are using External Identities (Azure B2C, Facebook etc) the Hack, which may not be supported, would be to add your External Identity record (child record to your Contact record) to the the other users Contact (i.e. reparent your External Identity to the other Contact), you should find that you can then log as the other user.
