Actually, I think what Julian proposed is the correct solution. However, it’s a different form from what you use to get your 26 fields.
So you define a form in your stories to clarify if the user meant a username or the customer name and then run another form to get the rest of your slots. In general it would look like this:
* user_uttered_name{"general_name":"Julian Gerhard"} <!----- This lets the bot know to ask which name ------!>
- name_form
- form{"name": "name_type_form"} <!----- This form asks: what sort of name is this? Returns either username or customer name and stores the appropriate name in the appropriate slot -----!>
- form{"name": null}
* WHATEVER_INTENT_YOU_NEED_TO_START_YOUR_26_FIELD_FORM
- 26-field_form
- form{"name":"26-field_form"} <!--- this form handles the rest of your slots. One of them (name) is pre-filled.
- form{"name": null}