Simbrief does not support the IFR and VFR keywords in its route string so there’s no proper way to plan those without manually removing them from the Simbrief route and manually adding them to the validation text as required.
Thus despite the existence of “VFR to IFR” and “IFR to VFR” entries in the Flight Rules drop-down, such flights cannot really be planned with Simbrief that I know of 
I have made a feature request:
Regards,
Tim