Code
T=ones(2,2,2)
T(3,3,3)=100
T(T==0)=NaN
%%// T(~T)=NaN would work too, but not a good practice as T is not logical
Or
T=ones(2,2,2)
T1 = NaN(3,3,3)
T(1:2,1:2,1:2) = T;
T1(3,3,3)=100
Or
T1 = NaN(3,3,3)
T1(1:2,1:2,1:2)=1;
T1(3,3,3)=100