Hello
Your knitting looks lovely.
I can see a yarn tail on each needle in your photo so the first thing is just to check that the 21 you have already worked are the correct ones. These should be with the right side of the work facing you and end up on the right needle with working yarn attached.
You use the same working yarn and add the new cast on stitches to the needle with these 21 stitches already worked, there will be no gap, just a continuation of the work done. After adding the new stitches your working yarn will be at the end of these, in exactly the right place for you to work the next stitch on the left needle so that you can work the other 21 stitches.
To cast on in the middle I would use the knitted cast on, there are lots of video tutorials if you don’t like the one I link.
Most of the videos will show how to knit cast on at the beginning of a project starting with a slip knot, just ignore the slip knot part. You will work into the stitches you already have on the needle. It’s just like a knit stitch but instead of slipping the old stitch off the left needle you leave it there and slip the new stitch from the right needle to the left, one stitch created. Work into the new stitch to cast on another.
One more thing. In your work you have right side facing. To cast on you need to turn the work (as though you got to the end of a row) so that you have wrong is facing and the 21 you have worked are on the left needle. Then cast on the number you need. Then turn again. Right side facing work your other 21.
I will add. I have not made a hooded top myself and I’m a little confused as to what is happening at this point where there seems to be a gap across the back neck. However I tend to just go along with what a pattern says and usually it all makes sense in the end.
Does this part get sewn up in the finishing?
I hope this helps you understand this part. Please do say if its not clear.