I am rendering to a texture that has some random values

I am rendering to it using point list

I am rendering points to some random texels and sometimes more than one point writes to the same texel

When this happens I want the values that the points write to add

At the same time I want that this value that is the addition of the values to replace the value that was already in the texel before calling the draw function

I am doing this now by using the clear render target function and then calling the draw method using blending.

Is there a faster way to do this without having to use the clear render target function?