Your question is not clear. 'Bob' is a string, and not a fieldname, and you do not refer to it at all in your example of what you are trying to do, so it is not clear what relevance your statement "I need to add more data elements to Bob" has.
In any case this code works for me without error:
>> data1 = struct('x',1:3,'y',4:6);
>> data2 = struct('x',0:2,'y',3:5);
>> b = struct('bob',{'hi'}, 'data',{data1});
>> b.data(2) = data2;
If you are getting an error or something unexpected, then you need to tell us exactly that message or output. We cannot read your computer screen, nor your mind.
Structures are accessed by indexing and fieldnames, which you seem to understand, so it is not clear what your problem is. Please tell us what you are trying to do, and exactly what result you expect.
You can read all about structures here:
