Is it possible for an airline pilot in case he would like to fly GA aircraft like c152/172 obtain the PPL license without going through the whole process?
Note: You did not specify a country. I will answer for EASA.
You don't need to obtain a PPL, you may use the MPL (Multi-Crew Pilot Licence) if you fulfill the requirements. From EASA Part-FCL (emphasis mine):
FCL.405.A MPL — Privileges
(a) The privileges of the holder of an MPL are to act as co-pilot in an aeroplane required to be operated with a co-pilot.
(b) The holder of an MPL may obtain the extra privileges of:
(1) the holder of a PPL(A), provided that the requirements for the PPL(A) specified in Subpart C are met;
(2) a CPL(A), provided that the requirements specified in FCL.325.A are met.
(c) The holder of an MPL shall have the privileges of his/her IR(A) limited to aeroplanes required to be operated with a co-pilot. The privileges of the IR(A) may be extended to single-pilot operations in aeroplanes, provided that the licence holder has completed the training necessary to act as PIC in single-pilot operations exercised solely by reference to instruments and passed the skill test of the IR(A) as a single-pilot.
You can find the requirements of Subpart C in the same PDF on page 145f.