RE: but files from Office Store aren't in the right format to import.
Hmmm that's strange... any particular custom visual? I just downloaded a copy of TableHeatmap from Office Store and didn't encounter anything wrong as custom visual was available in both pbix report sample and .pbiviz package version...
also had no real problem running the pbix sample first, then import the pbiviz package and rebuild the same scenario
in Power BI Desktop afterwards.
Anyway I'll give it a closer round check... we never know... 