What kind of user interaction are you looking for?
Creating a new geometry object allows entering field values in the Record pane.
If you want to use a custom form, you can create a bunch of objects first and then run a script which will locate objects with NULL values in specific fields and ask to enter data for them. You can create the form using Windows Forms, a .NET API.
As an extension of the above, you can wrap the script into an add-in, either compiled (DLL) or not (CS / VB / etc), and invoke it from the main menu (Tools - Add-ins - <your add-in>).
We don't currently offer much help for creating / hosting custom forms from within the system, but there are plans to change that, and we are always open for small incremental additions before we get to address the area for real. For example, we can extend the object model with means to control component windows, we can let add-ins respond to user-initiated events (like "a new object just got added"), etc.