how to blit a surface (no alpha) to ARGB, while the alpha data of ARGB surface keep untouched

all:
  I want to blit a RGB24 surface to an ARGB surface.for
example  pixel RGB(128,128,128)+ARGB(100,0,0,0) --->
ARGB(100,128,128,128). is there a way to implement it or I
have some misunderstanding
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