Magento does not accept different Name than sitemap.xml
It is very simple, please follow below step.
/app/code/core/Mage/Core/Model/File/Validator/AvailablePath.php copy this file and paste this in your local folder
/app/code/local/Mage/Core/Model/File/Validator/AvailablePath.php
find
if ($this->_availablePaths && !$this->_isValidByPaths($valuePathInfo, $this->_availablePaths, false)) {
$this->_error(self::NOT_AVAILABLE_PATH, $this->_value);
return false;
}
replace with
if ($this->_availablePaths && !$this->_isValidByPaths($valuePathInfo, $this->_availablePaths, false)) {
$this->_error(self::NOT_AVAILABLE_PATH, $this->_value);
return true;
}