Well reflections are controlled by the alpha channel in the texture but its not quite that simple....

If a model has been originally created to have reflections then altering the alpha channel will alter the amount of reflection but if it hasn`t then the alpha will instead control transparency. If the model is one of the "transparency" type then you can`t apply any reclection to it. Only the original modeller can alter the materials inside the model

Actually you *can* make some changes to model material using the MDLMat/MDLMatX programs from my beta page but this is advanced stuff that requires a working knowledge of FS materials (or a lot of patience for experimentation)