You would have to raise it all together to avoid artifacts where components join. If you set the brush strength to some large value (you can actually type in the box larger numbers than you can drag the slider to) then a single click will raise a lot in one go.
Alternatively you can export the heightmap to png and add to it in an image editor that supports 16-bit grayscale images (e.g. Photoshop), and then reimport it through the right-click in the landscape editor.