This script will fetch an Atom feed and insert rows on top in descending order. Make sure to replace with arbitrary URLs on 2 placeholders.

Apps Script to insert Atom Feed into Google Spreadsheet

function main() {
  var ss = SpreadsheetApp.openByUrl('YOUR SPREADSHEET URL COMES HERE');
  var sheet = ss.getSheets()[0];
  
  var property = PropertiesService.getDocumentProperties();
  var last_update = property.getProperty('last_update');
  last_update = last_update === null ? 0 : parseFloat(last_update);
  
  var feed = fetch('ATOM FEED URL COMES HERE');
  var items = getItems(feed);
  var i = items.length - 1;
  while (i > -1) {
    var item = items[i--];
    var date = new Date(item.getChildText('pubDate'));
    if (date.getTime() > last_update) {
      insertRow(item, sheet);
    }
  }
  property.setProperty('last_update', date.getTime());
}

function fetch(url) {
  var feed = UrlFetchApp.fetch(url).getContentText();
  return feed;
}

function getItems(feed) {
  var doc = XmlService.parse(feed);
  var root = doc.getRootElement();
  var channel = root.getChild('channel');
  var items = channel.getChildren('item');
  return items;
}

function insertRow(item, sheet) {
  var title = item.getChildText('title');
  var url = item.getChildText('link');
  var author = item.getChildText('author');
  var date = new Date(item.getChildText('pubDate'));
  sheet.insertRowBefore(2);
  sheet.getRange('B2:E2').setValues([[title, url, author, date.toLocaleString()]]);
}

Hello,

When I try to run the function I get the following error Cannot call method "getChildren" of null. for the following line
var items = channel.getChildren('item');

Any idea why? Do you think you could help with an input?
I'm a noob in terms of programing and have some difficulties solving this one.

Thanks in advance for your help!

FYI - I would suggest getScriptProperties() rather than getDocumentProperties() as the latter could be null https://developers.google.com/apps-script/reference/properties/properties-service#getDocumentProperties() (I ran into this myself when trying to implement your script)