While I think both answers are no, whether to demote Joseph and whether to call up Hoskins are separate questions. Even if I did the former I wouldn't do the latter. Yeah, Hoskins is on fire but you can't make decisions based on four weeks. If he had been in AAA for even part of last year, different story. Calling him up too fast could just end with a demotion for him too.
If Joseph flops fast my guess is they'll patch together 1B (Kendrick, Nava, Stassi) until June, with the bonus of keeping Altherr in the everyday line-up.