Hi. I see this thread is way too old, but I think I have the solution: B♭m9/C♯ is an enharmonic of C♯maj7add13, that is, it has the same notes but with a different chord name. C♯maj7add13 can also be considered as C♯maj7add6. The same way, C♯add13 is equivalent to C#6 and also to B♭m7/C♯.

The reason why we use to consider thirteenth instead of sixth or eleventh instead of fourth or ninth instead of second is that they have a whole octave in between, and thus the dissonance caused by the strange note (in our case, the sixth over the major seventh) is less.

Regards.