I do not know of a retrofit for ceiling mount but I used a system that just unscrewed the wall mounted shower head and replaced it with a two system shower. Mine is here and needed no extra screws anywhere. I got it years ago at Costco but this is a similar one http://www.signaturehardware.com/hawick-shower-system-riser-with-hand-shower.html?zmap=926285&c3ch=PLA&c3nid=GooglePLA&gclid=EAIaIQobChMIscnjrPju1wIViR2BCh3FFgL2EAQYAiABEgKPg_D_BwE

This might do the trickPDF Spec Sheet for 3-Way shower arm diverter.

The item is this: Shower diverter allows water to go to original shower head, the new hand-held, or both.

This one is at e-Faucets and goes right above the shower head. Is it a big rainshower head where you would have a hard time reaching anything attached right above the shower head? If not, this one would work for you. Delta Faucet U4929-SS-PK Universal 3-way shower arm diverter for handshower

Home Depot carries this second one, too. They seem to have at least a couple to chose from. Some have a grip for the hose on the hand-held, as well as the knob for turning to the various positions.

If you want just one OR the other one on, ask if they have a two-way valve, or just the three-way ones I have been posting. You may not want that ceiling shower fixture to be getting a small stream of water if the diverter stays in between with "both on" because that could mean cold drips falling down on you. Seems to me that a rainhead will occasionally drip cold water easily as it is, and giving it a slow feed of water might do that.

Yes. It's possible with a shower arm diverter but the diverter will be up high on the ceiling head pipe. Could be difficult to reach depending on height.