Unfortunately, there is no simple config option to add a BOM character to the beginning of a file.
There is a work around, but it is not pretty. See here for a hint (basically, if the files are < 5MB you can hack our CSV parser and generator to inject any text you want into the file). LMK if you need more info to make this work. Else, please let us know if this is a show stopper for you, so we can prioritize an enhancement to expose this via a simple config option.