Simulink does not have an IQ datatype, so it is difficult to know what you are asking for.
Simulink has a real + imaginary to complex block https://www.mathworks.com/help/simulink/slref/realimagtocomplex.html but that forms complex numbers.
The MATLAB / Simulink internal representation of complex numbers currently interweaves real and complex data; this is relatively new, with it not being all that many releases ago when complex numbers were represented by an array of real values and a seperate array of imaginary components.
I guess you could use the real+imaginary to complex block with constant imaginary part of 0; the result would in some meaningful sense be IQ data
