HLSL is neccessary to understand the script/code within a shader, but youcan't just go whapping any old HLSL shader into FPSC.
DBpro (and consequently FPSC) needs the syntax and formatting of the shader code to presented in a certain way in order for it to work.
For more information I'd advise reading some of the documentation on the DB boards about making shaders. You will be able to get alot of help there. There is a nice manual with DarkShader that explains some of the Key HLSL commands, but I'm not sure if I can post the manual here, as it is part of the DarkShader package (and therfore is probably protected by the licence.