well all float4x4 are variables

Maybe liitle example? PLEASE
F.e. code from render monkey and next "fixed" one to use in 3dgs

There is a list of shader variables in the manual. These are the variables that the engine can send to the shader. If you want to convert a shader, you should just rename the variables to the ones in that list. Usually this is very straightforward. For example, you could see a variable MWVP or world_view_proj, and you'd change that to the 3dgs name: matWorldViewProj. If you've got a little shader knowledge you can also guess the right variable from the way it is used in the shader.

It's not really possible to create a general list for conversion, because every shader (for a different engine/tool) will use different names for their variables. I don't know rendermonkey, but if it uses a single variable naming sheme, you could create such a list or even automate it.

I'd say just give it a try and when you get stuck, ask for help. Generally I rather help someone who has given it a try for themselves, than just doing all the work for them..

EDIT: one more hint for converting: generally, you can recognize a variable that has to be changed by the fact that it is not assigned a value:
float4x4 mwvp; // This is not initialized, so it must be set by the engine -> change it to matWorldViewProj.
float4 baseColor = {0.3f, 0.7f, 0.2f, 1.0f}; // A value is assigned so this variable doesn't have to be changed.

Sometimes, a variable is not assigned where it is defined, but where it is used:
int numTaps;

float4 mypixelshader(inTexcoord : TEXCOORD0)
{numTaps = 4; // Initialized here, this is clearly not a variable that is set by the engine.
blur(numTaps, inTexcoord);
}

Thanks Excessus. This is a very helpful answer. I even did not know that there is a shader variable list in the manual. I have to look for it.

That's some very usefull information, Excessus. Thank you! Maybe this could be worth an article in the wiki?