While indigo doesn't require that your fabric is mordanted, there is no adverse reaction it if it has been.
So if you have leftover fabric from your Natural Dye Kit that was mordanted, you can dye it in the indigo with no trouble at all.
In fact, if you're planning on overdyeing (i.e. you can take some of your blue dyed fabric and dye it in leftover osage to get green) it's even useful that it's already been mordanted.  More information on overdyeing can be found here.