This is an intresting question. I had to solve the same problem more than once. For example, to make semitransparent diffuse textures for 3d objects in PS Extended. There it seemed to be the only way
So one way is to use Filter Factory plugin. Just set alpha channel value to 255.
The other one is, as was mentioned above "duplicate the layer more than one". The only thing is: I'm afraid that to make 1% opaque pixel fully 100% it would take MUCH more than one duplicate. Somewhere around several hundreds
Here is how to do it right. Start recording an action. Record two steps: duplicate current layer and merge down. Stop recording. Now select these two steps and copy them another 10 times. All in all there must be 11 duplicate/merge steps. Exactly eleven, no less! This action will grow any semitransparent pixel to fully opaque.
Now about "to change there opacity gradually by hand". To reduce the opacity use eraser tool. There is no problem here.
But to grow the opacity there is no special tool in PS. The only way I found: Select your layer in the Layers panel. Go to history panel and set the source for History brush to the last history state. Take History brush and set it's mode to Behind. Start painting on your layer. Painted areas will grow their opacity.