Can a severe electrolyte imbalance cause an epilectic seizure in someone who doesn't have epilepsy? and if not then what can cause a seizure?

Yes: An electrolyte imbalance can cause a seizure. So can many other events that are not epileptic. When someone has a seizure, we first look for other causes before considering epilepsy as the chances for repeat seizures are very low when the cause is treated. If someone has a known seizure disorder, the same things can cause loss of control even when medication has been effective.