Please, I would like to buy Yamaha DGX 660 and I am wondering if it can emulate Hammond (B3...) organ?
It really depends on what you mean by 'emulate.'
The Hammond B3 (along with the A3 and C3) is a 'pitch wheel' instrument, which means that an electric motor turning a shaft connected to tiny toothed wheels is what generates the pitches you hear. Thus the vibration speed of any pitch can be represented by a whole number (the number of teeth on the wheel times the speed of revolution). In the 'real world,' including the world of electronic instruments, only the note A can be represented by a whole number, such as in the definition A440. All other pitches in a tempered scale are multiples of the 12th root of two times some given number between 1-12. Or to say it another way, a Hammond Organ (of that era) is NEVER in tune by tempered tuning standards. An electronic instrument which uses sampling can come close, but because of tiny variations in sampling frequency can never be exactly the same as the sound source. It will come close if done well, but will never be exactly the same.
So if by 'emulate' you mean 'approximate the sound of,' the answer is yes. If you mean 'match the pitches of exactly' the answer is no.