You can do it with a bit of custom code on the 'page submit code' setting on the order form (whichever page contains your gateway selection field). Create a hidden field, name it 'currency', and associate it with the currency order value. Give it a default value of 'KES'. Assuming the field that allows a choice of payment gateway is called 'gateway' and you want to force it to US Dollars if using Paypal, you would use the following code in the page submit code setting:
if (nbf_common::get_param($_POST, 'ctl_gatweay') == 'paypal')
{
$_POST['ctl_currency'] = 'USD';
$_REQUEST['ctl_currency'] = 'USD';
}