Hello Viktor,
I can't add it to Admin Panel -> Categories, because I have escort script and in categories is nothing from agencies.
When I look in Admin Panel -> Account Types -> Build Registration Form -> Agencies, there is also no field to add.
I tried to add this field in Admin Panel -> Registration Fields -> Add A Field but there is no Date type Availability!
I suspect this system field is hidden in escort script, and I can only change it in the script, but where?