If you only have Suction but no Sr, use empirical relations related to GSD of the soil or its plasticity indices. For example use the AASHTO guidelines here attaced.
You can use the RETC code and fit an equation for you texture class soil (estimate intial values using the neural network code Rosseta available in RETC code. After that you can see if it fit with some of your suction values and adjust (by hand) until it fits.