Ah ok then, for that you don't use the TPac tool. You use the Bannerlord modding tools (you can download them via Steam).
After launching the Bannerlord modding tools, go to file -> Create New Model and name it something. Then restart the modding tools (no need to save anything) and make sure only your mod is selected. Then open the resource browser (this button
), go to your mod folder there, then assets and right-click and choose "Import new access". Then import the file you want to put into the game. If the name is the same as the vanilla one it should overwrite as long as your mod comes later in the loading order.
If you have any trouble using the tools to create the module/using asset browser, check this tutorial
at minute 5:50. It covers it pretty well.