It is possible, but requires a little PHP programming on the advanced tab of your order form. You also need to create a product record for the donation - you can give it a 'one-off' price of 1.00 (or whatever, doesn't really matter because you will be overriding the amount, but it should be more than zero). Also make sure the 'payment frequency code' on the 'order' tab of your order form is set to 'AA'.
Suppose you have a numeric field on your form called donation_amount, which holds the total amount to be billed (also assuming that no VAT is going to be added), you could use the following code for the 'submit code' setting.
$orders = array();
$order = array();
$order['net_price'] = format_number(mosGetParam($_POST, 'donation_amount'), 2);
$order['tax_amount'] = 0;
$order['product_name'] = "Donation";
$order['quantity'] = 1;
$order['product_id'] = 1; //Change this to whatever your actual product ID is
$orders = $order;