like this?

	const D3DXMATRIX robertx = D3DXMATRIX(1, 0, 0, 0, 0, -1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1);
	

I don't see why not. Naturally, if you multiply that matrix by anything, that other thing will also have to be a 4 dimensional entity. So if you want to use it to transform a 2d vector, then that vector will also have to be stored in the first two components of a 4d vector with the fourth component of that vector being 1.

Though depending on what you are using this for that last part might not be necessary. You've been sparse on details and there could be complications or trade-offs.