Please see http://www.coade.com/uploads/caesar%20ii/webinars/may-2011-webinar.pdf
under title "Stiffness Factor for Friction".

I think there the key sentence is "Use it as a tuning parameter in forensic engineering.".

Vs this sentence, your wording "for achieving frequency in modal run" is unclear. You tried to achieve compliance with an observed frequency or you tried to achieve a target frequency considering nonlinear effect of friction? For the last one I think the following post gives a fair answer
http://forums.coade.com/ubbthreads/ubbthreads.php?ubb=showflat&Number=47517