You'll have to export your hand histories from PT3 and reimport them into a new HEM database.
To export from PT3, go to Database menu> Database Management, then choose your main DB and click the export button at the bottom.
To import in HEM it's pretty straight-forward, just go to the Import tab, and import from folder, then browse for wherever you exported your HHs to.
Keep in mind you can only import 5k hands into the trial version of HEM.