Hi
as u have noticed XSI doesn't have the traditional material editor..instead u work with the render tree or the material ,texture ect property pages..
if u want to blend e.g the diffuse with a texture instead of plugin the texture node direct to diffuse use a mix 2 color node and plug this node to diffuse..now u can adjust the blend through the mix 2 color..
u can do the same with ambient and specular too..u have the freedom to use the same texture for all of them or different ones..
a quick way to do this is select your object then
GET>MATERIAL>PHONG,BLINN ect..
GET> TEXTURE>IMAGE..
open the render tree and u'll see the e.g phong shader with 2 different mix 2 color nodes, each one for diffuse and ambient ..
hope it helps..
george