Ejaculation without orgasm: is this possible to do it at will without external stimulation?
Yes, it is possible. It is all in the brain. One can be sufficiently aroused to ejaculate without any contact.

The most obvious example of this is the case of nocturnal emission


  A nocturnal emission, informally known as a wet dream, is a spontaneous orgasm during sleep that includes ejaculation for a male, or vaginal wetness or an orgasm (or both) for a female. Nocturnal emissions are most common during adolescence and early young adult years, but they may happen any time after puberty. It is possible for men to wake up during a wet dream or simply to sleep through it, but for women, some researchers have added the requirement that she should also awaken during the orgasm and perceive that the orgasm happened before it counts as a wet dream. Vaginal lubrication alone does not mean that the female had an orgasm.


Depending by what you mean by "without such external stimulation", you might want to read about premature ejaculation


  Premature ejaculation (PE) occurs when a man experiences orgasm and expels semen soon after sexual activity and with minimal penile stimulation. It has also been called early ejaculation, rapid ejaculation, rapid climax, premature climax, and (historically) ejaculatio praecox. There is no uniform cut-off defining "premature", but a consensus of experts at the International Society for Sexual Medicine endorsed a definition including "ejaculation which always or nearly always occurs prior to or within about one minute".