Pay your attention to the "Load as" panel under the Channels list. It lets us decide how to handle the existing selection on loading the new one.
By default the script suggests the "New Selection" option, which means replacing the existing selection. However, by using the radio-buttons you can add, subtract, or intersect the loaded selection with the previous one.
The selected mode apply to the following commands:
- Load Selected Channel.
- Load Layer Mask.
- Load Vector Mask.
- Load Smart Filter Mask.
- Load Layer Opacity.
In the example shown on the screenshot, the Green channel is subtracted from the existing rectangular selection.