I tried newTF.contents="bla bla..." and it gets into the newTF.
this is my updated code:
<pre>var myDoc = app.activeDocument;
var myPStmuna = myDoc.paragraphStyles.item("תמונה");
var myPScomment = myDoc.paragraphStyles.item("הערה");
for(i=0;i<myDoc.pages.length;i++){
  myPage=myDoc.pages.item(i);
  tf=myPage.textFrames.item(0);
  for(g=tf.paragraphs.length-1;g>=0;g--){
    if(tf.paragraphs.length>0){
      myPara=tf.paragraphs.item(g);
      if(myPara.appliedParagraphStyle == myPStmuna){
        newTF=myPage.textFrames.add();
        myBounds = myGetBounds(myDoc,myPage);
        newTF.geometricBounds = myBounds;
        newTF.contents="bla bla\r";        
        myPara.move(LocationOptions.atEnd,newTF);
        
      }
    }else{
      break;
    }
  }
}
function myGetBounds(myDocument, myPage){
var myWidth = myDocument.documentPreferences.pageWidth;
var myHeight = myDocument.documentPreferences.pageHeight;
var myX1 = myPage.marginPreferences.left;
var myY1 = myPage.marginPreferences.top;
var myX2 = myWidth - myPage.marginPreferences.right;
var myY2 = myHeight - myPage.marginPreferences.bottom;
return [myY1, myX1, myY2, myX2];
}</pre>