Yes. Wouldn't be too hard. You would simply have to edit the bank code. Find this part
//if user is trying to deposit more money than they have
if ($_POST['amount'] > $row['money']) {
$context['shop_buy_message'] = $txt['shop_dont_have_much'];
//if trying to deposit less than 0
} elseif ($_POST['amount'] <= 0) {
$context['shop_buy_message'] = $txt['shop_no_negative'];
}
Add elseif ($_POST['amount'] <= 200) {
$context['shop_buy_message'] = "You must desposit at least 200 credits";
}
And yay! Actually, it would be best to edit the shop.english.php file or whatever its called with a $txt['shop_less_200'] = "You must desposit at least 200 credits";
line and change it to elseif ($_POST['amount'] <= 200) {
$context['shop_buy_message'] = $txt['shop_less_200'];
}