Generate a layer mask which contains all transparency info of the layer. When you add a layer mask you get an opportunity to initialize the mask with existing layer alpha (=all cumulated transparency data)
An example: A layer with solid fill and a couple of strokes with the eraser. Layer > Mask > Add layer mask dialog is opened and option transfer the alpha channel is selected:
In the next image the fill color is again solid red, but all transparency is now in the layer mask (see the grey X):
Now you can select the mask icon and paint to the mask. Paint full black for full transparency or paint full white for 100% opacity. Paint grey for something between. Paint with transparent brush and let the brush have some blending mode other than normal for relative results. To be effective this needs knowledge of the blending modes.