Differential driver IBIS model

Hi all, I am new to SI and SigXplorer as well and I have to do simulations on a DDRII chip attached to a freescale MPC8313 placed on my board. I have been playing with differential pairs in SigXplorer and have seen that there are models for differential output buffers and differential input buffers, the problem comes out with the IBIS models supplied by the vendors.For example let's focus on the differential CLK line,the DDRII IBIS model comes with a differential input model but the MPC8313 IBIS model doesn´t have a differential output buffer for the clock, instead you have two single ended models each one for clk+ and clk-. From hardware point of view a differential signal consists of two pins having the same models, but opposite signal phase. And my question is, is there a way in SigXplorer to take two single ended signals and invert or change the phase of one signal respect from the other, is this the right way from a simulation point of view? In "Stimulus Edit" I cannot change the default stimulus signal and I don´t have a way to create a custom stimulus. I need to see the eye opening and reflections but cannot with two single ended models. Thank's in advance Regards.