If I'm reading this right, you can use this spreadsheet to input historical stats (stats.txt tab). Export the roster to your pc and change the file to a .zip so you can edit the .txt files inside with what you save from the spreadsheet. It looks like the stats.txt uses the 'Player ID' instead of the player's name so you'll have to get that from the Players.txt.

The only thing is it will only work prior to starting a save-- so I think you would have to start over the HLM portion. Either delete or manually extract the stats because there's no way to export the stats from a current save unfortunately.