I found lots of studies about this but not regarding nursing training. Only at the end did this paper mentioned training nurses in this technique:
Mansour SE et al (2015) Effect of Nurses Using for P6 Acupressure on Nausea, Vomiting and Retching in Women with Hyperemesis Gravidarum Journal of Nursing and Health Science Volume 4, Issue 4 Ver. II (Jul. - Aug)
And this was regarding the Royal College of Midwives, but it discussed attitudes rather than training needed:
Soltani, H., & Taylor, G. M. (2003). Changing attitudes and perceptions to hyperemesis gravidarum. RCM midwives: the official journal of the Royal College of Midwives, 6(12), 520-524.