PDF spec does not define 'annotation creator' as a standard property.
Most PDF annotators store 'annotation creator' under "T" entry in markup-dictionaries.
you can call marup.SetTitle("User name") and markup.GetTitile() to set/get info you are looking for.
Alternatively you can set the extra info as custom key/value pairs via SDF API. For example
annot.GetSDFObj().PutText("MyCustomKey", "My user name");
Obj val = annot.GetSDFObj().FindObj(MyCustomKey");
if (val != null) {
string val = val.GetAsPDFText();
...
}
for more sample code please see SDFTest sample.