The cloud textures included with Vue are procedural materials, not image map textures, so no, you can't use them.

What you could do is use Terragen, another landscape program that is freeware, render Terragen clouds ( white clouds against black sky) and use this as a texture for your alpha planes, but you need to be a little familiar with Terragen for this. There wa a free flaming pear plugin for Photoshop called "cumulus" that is also great for this, but they don't give it anymore, nor do they sell it. Maybe you can find it in an old edition of computer arts, or some photoshop oriented magazine.