How can I generate a multipage text document from a single page template in google-apps-script?

Well Serge, haven't you tried the appendParagraph and other appends of the DocumentBodySection?

I'd do it like this:

function mergeDocs() {
  var docIDs = ['list-of','documents','ids','you should have somehow'];
  var baseDoc = DocumentApp.openById(docIDs[0]);
  var body = baseDoc.getActiveSection();

  for( var i = 1; i < docIDs.length; ++i ) {
    var otherBody = DocumentApp.openById(docIDs[i]).getActiveSection();
    var totalElements = otherBody.getNumChildren();
    for( var j = 0; j < totalElements; ++j ) {
      var element = otherBody.getChild(j).copy();
      var type = element.getType();
      if( type == DocumentApp.ElementType.PARAGRAPH )
        body.appendParagraph(element);
      else if( type == DocumentApp.ElementType.TABLE )
        body.appendTable(element);
      else if( type == DocumentApp.ElementType.LIST_ITEM )
        body.appendListItem(element);
      else
        throw new Error("According to the doc this type couldn't appear in the body: "+type);
    }
  }
}