It is not allowed to add new fields to a Structured DataType. However, a subtype can be created having additional fields. It depends on the usage of the DataType, whether the subtype can be used. If it is used in another structured DataType not stating, that subtypes are allowed, this does not work (see 7.9). For Variables, it can be specified if subtypes of the DataType are allowed. If not, this can also not be used. For Method arguments, the meta data may define whether subtypes are allowed, but the Method cannot be changed requiring the subtype. Therefore, in those cases ideally the structure would already have an integrated extension mechanism (see 7.9).

If this is not the case, a new subtype needs to be created and all its usages replaces, which might be very cumbersome, specifically if the DataType has been used directly or indirectly in various places.