You can do that by declaring the first elements in an array:

const elements = [

  <p>hello</p>,

  <p>world</p>,

]

And then pushing whatever you want to the array:

elements.push(<p>Additionnal</p>)

You can then use this element using {elements} anywhere you want.

Bear in mind that this array notation would require you to use a key parameter in each of these childs, so something like this would be better:

const elements = [

  <p key=1>hello</p>,

  <p key=2>world</p>,

]

elements.push(<p key=3>additionnal</p>)