Will it harm to charging my iphone with usb port from imac's Keyboard for the first charge?

Powering peripherals through USB - Apple Support

Here is the official page from Apple, which might be helpful.

To answer your why for my case - I want to play music from my iPhone at work. I sync my phone to iTunes on my home computer to avoid dumping all my personal info on my work computer.

And I don't have enough USB ports on my MacBook to add my Phone.

I can't find the specific recommendations from Apple - that's what I was looking for from this thread. I'll keep searching for it.

It certainly doesn't drain my battery=. I have the aluminum keyboard and it charges my iPhone fine, but don't know if it damages the battery.

Won't damage your phone, but also won't provide enough power to charge your phone.

It could also drain your battery. Why do you want to do something that Apple specifically recommends you not do?

If you have a mac with the white wired-keyboard, the keyboard does not put out enough power to charge the phone. If you have an aluminum wired-keyboard, the keyboard will provide up to 500mA to one device (enough to charge the phone), and 100mA to another device (600mA total). If there are two devices asking for >100mA, one will get it, one won't.

So, if you have a mouse and your iPhone plugged in, it'll probably work. Two iPhones, an iPhone and a pen-tablet or portable hard disk, it will probably fail.

Note that if you connect an iPhone directly into the Mac's rear USB port, it will be permitted up to 1000mA and will go into rapid-charge mode.