I have a tub and shower with no means of placing bottles of wash or shampoos except for around the edge of the tub. I tried to put in a make-shift shelf tower but it does not fit. I wanted to put porcelain shelves in the corners but I need to do it over existing tile. I have done it before without removing the original tile but it was about 9 years ago and I can't remember how I did it. I tried to remove some of the existing tile to place one shelf but managed to pull out Sheetrock that now has to be replaced. I am not going to ruin any more of the tile or wall.
I would say it is too late to add these. Another alternative is to use a shower caddy. They are fairly common, and can hold soaps, shampoos, etc.
If you really wanted to go this route, you may end up having to re-tile the entire shower.
You should be able to drill holes in the tile (there are special drill bits for that) and then mount the shelf via anchors. Make sure to use tub and tile caulk around the edge of the new fixture once it is installed.