Question: For a normal 5 watt handy working in VHF or UHF freq with 50 ohm antenna , is it possible to use a same length coax cable instead of a stub antenna, so as to work like a crude antenna. Will this only result in extra capacitance/ VSWR damaging the system ?

Answer: 
You can remove the shielding of the coax leaving only the center conductor, but as mentioned in the comments the length will be off since this is way less then 1/4 wavelength and there is nothing added to increase the inductance. if the coax is 1/4 wavelength then it will work great.
Grid dip the coax to quarter wave, then put half wave wire on the center conductor end. Should work fine but clunky. Why not buy one of the longer antennas made for walkies?