Can an image of the Buddha be used as an ikon of St. Ioasaph?
To my knowledge, no. An iconographer must use an portrait of the saint as such has been preserved in the memory of the Church; he cannot use anything else as a model. For instance, an iconographer is not permitted to use his sister as a model for an icon of the Theotokos or of another female saint. Likewise, the Buddha cannot be used as a model for an icon of St. Iosaph.
And, if you're thinking of just taking an image of the Buddha and calling it an icon of St. Ioasaph, that's even more intolerable. The only thing you can use as an icon of St. Ioasaph is an icon of St. Ioasaph. If such an icon doesn't exist, I guess your only options are to have an iconographer paint one for you or just go without.