There isn't direct support for this in XLSForm but you can do it with a modification to the converted XML as described in Generate multiple choice alternatives, based on the answers in a repeat - #4 by LN.
There is a proposal to add support for this in XLSForm at https://github.com/XLSForm/pyxform/issues/38 and it looks like @Ukang_a_Dickson and team will be addressing it soon.