I don't see that happening. Joanna has shown that she does not forgive easily. Especially, when you dig into her past and bring it up on a national platform (see Adriana). Joanna, also, knows how to get the audience on her side. She'll stay loyal to Lisa like she did Lea. She knows to stick close to the Queen Bees w/o stabbing them in the back and still reaping the benefits.
And Brandi would only play nice for so long before she makes a stab for Joanna again. Plus, Brandi hates women that are younger and hotter than her. While I don't care for Joanna, she is 20x better looking than Brandi.
Joanna joining would be bad for Brandi (and Yolanda too) because she'd be the one that would go toe to toe with Brandi and not feel any remorse. Joanna is actually more ruthless than Brandi IMO. She has that European mentality where she doesn't show emotion unlike Brandi, who shows nothing but emotion. Joanna would trample all over her.