Hello,

We are afraid that you cannot make new entities or relation types even to exsting diagrams.

Only thing I can suggest is to use "Customized Icon". What it does is that you can specify an image for each Stereotype. By adding a stereotype to model, the model element will be displayed as a specified image. This isn't exactly what you are looking for but could help if you want new entities with specific notation that Astah doesn't have. This feature is available for Astah Professional only though.

Reference: How to use customized Icon P.118 of Astah Professional Reference Manual

Please let us know if you have any questions about how to set customized icons..etc.

Thanks,
SJ