As with all tile, if its not being installed on a hard flat surface then you need to use cement board to install the tile onto it. You never install tile directly to a wood floor, as it will crack and shift prematurely. To keep tile from cracking, you have to prevent shifting of the tile and the best way to accomplish this is with a solid underlayment, cement board.
As long as the tile is not severely broken in pieces it will be okay. Then lay the cement board over the tile and screw to the floor with masonry screws, not nails. Then install the tile over the cement board. If you dont use the cement board the tile will crack and the glue will not adhere since the exisiting tile is probably glossy and unlevel due to the grout lines between each tile.
People usually do not install tile to exisitng tile because the added stackup will cause door clearance and threshold problems that are not feasible to fix.
People usually install tile over exisiting tile if its the type of tile, like was in my house, that was used in the 1950's. These tiny tiles are practically cemented to the floor and it feels as if you need a jack hammer to remove.
In short, check your door/threshold clearances, it will raise your floor probably 1/2"-5/8" based on 1/4" cement board, 1/4" tile, and some thickness for glue.