Yes, it is supported:
from the data sheet:
FlexStack-Plus is backward compatible with 2960-S and FlexStack. Mixing 2960-X and 2960-S members in a stack is supported. When the stack membership is mixed with 2960-X and 2960-S models, FlexStack-Plus falls back to FlexStack capabilities of 40Gbps stack bandwidth and four members maximum per stack.