Been ages since I used Bryce, but similar to what  rj001 says, sort of, if you take a photo of a real clouds that would look ok on mountains, so you need the clouds ot have been on far away  to get correct shadows etc

CLouds that are up close show too much of their bottoms and shadow and would look fake, you'd need ones obviously far away that would fit into a far off scene.

you can edit that picture in a 2d editor (Photoshop, paint shop pro etc), crop out just the clouds to leave the rest transparent, save as a .tif or similar format with transparency (.png or .psd perhaps)and use as a picture image.

Put that in front of your mountains, so you have good looking clouds in front of the mountains.

Make sure you set the Sun in Bryce so it roughly fits the angle of light on the photo clouds!

Adding a little depth of field of blur can hide imperfections
:)

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