How do you add a hand held spray head to existing shower head?

Swap out current showerhead for a hand held model. The link provided is just an example of what to look for.

https://www.lowes.com/pd/Delta-In2ition-Chrome-Shower-Head/50297997

You can buy them at Home Depot. I bought a shower head that was a 2-in-1 shower head, it was a shower head that was held on by a magnet so you could remove it & use it as a hand held. I saw another option to buy just the hand held where you remove the existing shower head to insert the hand held attachment & replace the existing shower head.

You will need a new showerhead. I don't know of any way to convert a standard showeeheas

You just screw the old one off and screw the new one on.

Have had a Shower Massage for about 35 yrs. and I love it. They don't seem difficult to install.

You just screw old one off and screw the new one on.

Seems you want both and not just the hand held. I also have both. I like the showerhead, my son like the hand held (he's too tall for the showerhead) What you are looking for is a hand held shower T-diverter so you can have your regular showerhead and a hand held at the same time. The diverter has a lever to switch between the showerhead or hand held. I know Walmart sells them starting at $9 +. Ace Hardware, Menards, or Lowes also. Here is a link for Home Depot as an example.

http://www.homedepot.com/p/Delta-Shower-Arm-Diverter-for-Handshower-in-Chrome-U4922-PK/203124751

I have an existing tub faucet unit that is now in the wall. Is there a way I can add just a hand held sprayer without using the converter unit that attaches to the faucet. I would like for it to sit on the deck of the tub

You can simply do it. Follow this article's instructions on how to add Second Shower Head to the bathroom. Hopefully, your question will be solved.

https://showeradvise.com/how-to-add-a-second-shower-head/