HI subseageorge, Looks like you are trying to resurrect a post that has been in suspended animation for nearly six years, but we are soon in the NE about to enter mulberry season...
I suspect that the idea of adding some banana to the must is to give the must some better mouthfeel and if that is the case then you could add some oak or tannin and that would have a similar effect. I should say that I have made mulberry wine only a couple of times (and never with added banana ) and have never had a problem with mouthfeel.