The #_exchange_rates table uses only the rate_buy field as the exchange rate in FA v2.3. The rate_sell field is populated with the same value using line 74 in gl/includes/db/gl_db_rates.inc:
add_exchange_rate($curr_code, $date_, $ex_rate, $ex_rate);
You might want to directly populate the said table from your records - either a CSV import or using INSERT sql statements generated in a spreadsheet.
The structure of the said table is listed here for reference:
CREATE TABLE `0_exchange_rates` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`curr_code` char(3) NOT NULL DEFAULT '',
`rate_buy` double NOT NULL DEFAULT '0',
`rate_sell` double NOT NULL DEFAULT '0',
`date_` date NOT NULL DEFAULT '0000-00-00',
PRIMARY KEY (`id`),
UNIQUE KEY `curr_code` (`curr_code`,`date_`)
) ENGINE=MyISAM;