GECK can't esp'ify. Use FNVEdit, select file header in left panel, mouse over "ESM" to the right of "Record Flags", hit F2, untick "ESM", close, save, change file extension to esp. Any plugins with that ESM need to also be opened during the session and simply change the '.ESM' to '.esp' in the masterlist(s). Reloading all should then work out with the former ESM master as an esp.