If you need the Bb part in (presumably) treble clef alongside a C part in bass clef, I think it is best to have 2 different players, and just copy all music from one to the other. You don’t mention it, but maybe you also want the bass clef part transposed down an octave, in which case a duplicate player is probably the best choice. Otherwise you lose the original.
Apart from that, you can always insert a bass clef in an existing part (shift-C, then ‘f’ and return), and set the layout to print in Concert Pitch: Layout Options (select layout in right hand column) → Players → switch ‘Transposing layout’ off.
In that case, be prepared, when going to print, for a very useful warning that the layout you’re about to print is set to print non-transposing. If you create a separate non-transposing bass clef player however, you won’t see this.