'Orange Is the New Black' Star -- I'd Happily Be Caitlyn Jenner's Prison Bitch

“Orange Is the New Black” newcomer Ruby Rose is already casting another new addition for the hit show -- none other than Caitlyn Jenner ... who she believes would be queen bitch of the cellblock.

Ruby is an Aussie actress/model who's playing Piper's new love interest in Season 3 of 'OITNB' -- but she told our photog ... there's an opening for Caitlyn at Litchfield Prison, if she wants it.

You gotta hear Ruby describe it ... she's pretty much got the whole role written out already.