You should be able to add the deprecated Dashboard Designer portlet to a page in your 8.2 UMP. You can then import the 6.2 custom dashboard into this portlet.
Publish the customer dashboard from the Dashboard Designer and then deactivate and activate the wasp probe to migrate the imported 6.2 dashboard into the new HTML-5 Dashboard portlet.
The only caveat is that the name of the custom dashboard that you import/publish must be different from the names of any existing HTML-5 dashboards.