I'm assuming you mean you want to insert 500 entries in the admin panel for one Masquerade field?
Masquerade "drop-down" is not a true infinite-choice field. It's just one field with an in:
validation, where each possible option is listed separated by a space.
You can manually insert the in:
rule in the database. Be mindful of the maximum column length. You probably won't be able to insert 500 items. The max length of the validation column is probably just 255.
If you have many options, it would make more sense to extend the extension from another extension and customize the options there. You can extend the Serializer and Validator using Flarum events and return your comma-separated 500 options in there.