I think the best way to do it it's working with macros. You can work with an Excel file and then, convert it to the .txt format by using macros. A .txt is more suitable to be later worked as .app.
However, you can also do it by using Copy/Paste.
1. Open the .app in a Note Pad, so you can work with .txt file.
2. Copy the .txt data in the Excel file.
3. Then, it's just working with Data > Text to Columns to format the file. Then, you can work in Excel.
Then, you will have to put the Excel file in a .txt, so you will have to follow the data files requirements. You still can do this manually, but you will have to pay attention to the format (FILE_FORMAT=11.1.2, !VERSION=11.1.3609, !CUSTOM_ORDER=Custom1;Custom2;Custom3;Custom4, !APPLICATION_SETTINGS, !CURRENCIES, etc., as it is the Admin Guide). It will also depend on the HFM version you're working with, specially when working with customs.
I received a good document about a couple of months ago that can be helpful, so if you send me an email address, I'll forward it.