Alpha blending in color index mode.

Opengl documentation says, it is impossible to implement alpha blending in color index mode.

I am using Colorindex mode, for getting better performance.
and I have no choice other than color index mode.

Are there any another way to implement alpha blending keeping color index mode. Please suggest.

"I am using Colorindex mode, for getting better performance. "
what truly-ancient silicon are you using?!!?!

Think about it: alphablending of palettes. The perfect hardware would have to go through all palette entries and find the best matching entry (nearest-distance-of-color) on every RGB result.
So, no.