I would like to know if it's possible to do that with UNITY

I work for a furniture seller.

I have a important 3D project for him (already developed under WireFusion)

I would like know if I can do with your soft :

In a Database in a preview window of unity empty

call furniture linked to a database (.c4d or.wrml),

choose the database of textures (cushions, wood finish)

see the rendering of these choices,

then export a jpeg of the result.

Yes it is all possible

You can now stream assets on the fly in unity pro. Not sure about c4d or wrml though…

These would also be streamed as asset packs (e.g. asset pack wood).

If the assets have been streamed, you can use them right away.

You can make a screenshot, or render a cameras view onto a texture…Saving it to a jpeg probably needs a simple C# script.