If you have a naked HTMLElement, there's nothing Angular-y you can do. NgTemplateOutlet expected a TemplateRef, not a vanila HTMLElement object.
To insert an HTML Element into the DOM, simply use the appropriate DOM method for that. Which one you'll use entirely depends on the place where you want to use it.
For example, you could do
const wrapper = document.querySelector('div.wrapper')
wrapper.append(el) // pass in your element
You can also grab the reference to the wrapper in a more Angular way, by adding an Angular reference in the template on the element you want to select,
<div class="wrapper" #wrapper></div>
and using ViewChild decorator on a property where you want Angular to assign the ElementRef to.
@ViewChild('wrapper', { static: true, read: ElementRef })
public wrapperElRef: ElementRef<HTMLDivElement>
Once the view is initialized (you'll be notified when this happens via the ngAfterViewInit hook), you can grab the HTML element within by accessing `.nativeElement:
this.wrapperElRef.nativeElement //: HTMLDivElement
You can now do
this.wrapperElRef.nativeElement.append(elem)
That said, unless you know what you're doing (usually wrapping an existing library or integrating with an existing large system that you cannot yet refactor completely)