Wikipedia suggests that the origin of the j in marijuana is a bit more obscure than that:

The origin of the word “marijuana” foreshadowed its current use. Historically, the earliest and most numerous group of users in the Americas were slaves from western Central Africa (modern Gabon to Angola. Their words for cannabis are now used in nearly all the places they (involuntarily) ended up during the 1700s and 1800s…Most notably, in Central America, the Kimbundu (Angolan) word mariamba became the Spanish word marihuana.

The word “marijuana” as we know it today did not appear until 1846 in Farmacopea Mexicana, though it was spelled “mariguana”. In most following instances, the word was spelled marihuana. In Chilean Spanish, mariguanza is the dance of a shaman in an altered state of consciousness.