How to apply an alpha map to a layer mask?

I have a render I saved as a TIF, which has an alpha map included. Some parts of the map are semi-transparent, and I'd like to apply that to the image, instead of trying to recreate it with brushes. But I can't copy and paste the layer onto the mask. Anyone know if I can apply the map to the mask?

I personaly don't know how exactly it is done but i do know that there is a panel that lets you select which channel to use.

My understanding is that you paste the image in as a layer, go to the tab that is for the channels (on the same control panel that you select the layer on, its the second tab, between layers and paths), turn off the RGB and select the Alpha chanel.

This is where my knowledge gets fuzzy, i think you can choose to select the area somehow and copy it as a mask or some such.

Hopefuly i led you in the right direction. If no one answers between now and the next 8 hours, i'll have a gander when i get home.

Thanks.
I have the channel, and can convert it to an image. But, I need it as a layer mask on another layer.

hmm, beyond me sorry, never tried to do that. I'm sure there are tuts around for this type of problem, or someone here may have experimented with it.