The flatfile is in binary, so no, you can;t use excel. The only thing you can use is a hexadecimal editor.
To edit the flatfile, you can do a search though by looking for a string. To edit a player account then, you do a string search for the player's name. You then need to know the format of what immediately follows in order to edit the hexadecimal that corresponds to the value of the player's current prestige. Once that is done, you save the file, then restart the server. Sounds simple.
Always make a backup first, and if you make a mistake, you'll know soon enough.
The binary in the flatfile is stored in reverse pairs, and must be converted from hexadecimal to decimal, and then back again.
eg. 00 00 A1 45 in the flat file, has its digits reversed to 00 00 1A 54. Converting this to decimal gives us 6,740. Lets suppose we want to add on 5,000 prerstige. 6740 + 5000 = 11, 740. Converting back to hexadecimal gives us 00 00 2D DC. We then swap the digits back again to give us 00 00 D2 CD. These then are the hexadecimal numbers we use to overwrite the original hex humbers. You can use the Windows calculator for conversion. The tricky part is just making sure you have found the right hexadecimal numbers in the first place. If you havent, then you'll be editing something.