Dear DSP post are not CSS post DSP post were given by last govt by of PPP on the sufarish quota.and CM have power to directly appoint DSP. but now days it is not possible.
Which DSP job given by CM is very low promotion structure so you should prepare for competitive exam As like css.In css (PSP) group very fast promotions when you become ASP after doing 5 year jobs you will become SP after 12 became SSP 17 year DIG ANd after 27 years you will became AIG bps 21.

And 2nd step for join civil service of following group
PSP
PAS
FSP
IN css have 10% quota of above group for armed force first you join pak army/navy/air force through commission. when you do 3 year job in armed force after that when you go to join civil service they will take you only interview nothing else.you will skip any written test of css.because ISSB equal to written test.