I'm not familiar with SDKs like Unity or UDK that much, so i can't check this offhand. Do general purpose engines allow users to create custom uniform variables? The way i see it, and the way i have implemented it in an engine i'm writing to learn 3D, is that there is a "set" of uniforms provided by the engine and if you want to write a custom shader then you utilize uniforms you need to create a wanted effect.

Now, the thing is, first of all i'm not an artist, second of all, i didn't have a chance to create complex scenes yet. So my question is, is it common practice to define variables that the engine provides and only allow the user to work with what they're given?

Allowing users to add custom programs and use them where they want is not hard, but i have issues imagining how you'd go about doing the same for uniforms.