AC Adaptor converted in USB charger, is it safe?
It can be safe, and it can be logical, but based on your description, it is neither. By "AC adapter" I am assuming you are talking about a regulated wall pack (wall wart, as some people say)...

Every device has different power requirements: voltage level and current input. It is becoming easier to mix things now that so many devices charge with USB like cell phones, tablets, etc. USB is 5V DC, and can have a wide range of currents. First, you have to make sure that the AC adapter has a 5V output, and not some other value like 6V or 12V DC. Next, you have to see what the current output from the adapter is. It may well be higher than your intended device was designed to handle. It also might not supply enough current to power the device. This information should be printed on a small sticker on the side of the AC adapter. The power requirements for your device should be in the operator's manual.

On top of that, a USB cable has at least 4 wires in it. A power cable typically only has 2, unless the AC adapter has additional sensor wires in it. Yes, typically a red wire is V+, and a black, white, or uncoated wire will be ground. But this is not always the case! You would really need to test each wire with a voltmeter to determine what the correct wires are.

A good piece of electronic equipment should have protection from over current, over voltage, and reverse voltage, but not everything does. If you apply the power backward or use too high of a voltage/current, you could easily damage the circuitry or battery you are trying to charge. Also, not everything has an internal battery charger circuit, some of them rely on the circuit held within the AC adapter designed for that device. If you connect it to something else, you could damage the battery or cause it to overheat and explode.

Lastly, I don't understand how you plan to use an AC adapter as part of a "transfer cable." That doesn't make any sense.