Isn't this just unexpected? Softimage has just released Softimage|XSI EXP, a fully featured version of its model/animation toolset to create custom content for Half-Life 2. You can grab the 161mb download from FileShack. Using EXP you can create and animate new characters, level props, etc. The Softimage website has tutorials available.

Whether you are a 3-D professional, a novice or just curious, the XSI EXP for Half-Life 2 guides you through all the features and functionality in SOFTIMAGE|XSI, and lets you actually output Half-Life 2 characters and models. From polygon modeling and lightning fast Subdivision Surfaces, to nonlinear animation mixing and interactive simulation and rendering, it is all here.