You can add custom fields to your Bitrix24 product catalog in CRM>Settings>Product Properties - click "add" tab to add new fields:
In the product catalog property parameters form - choose new field type: string, number, list, etc.
For List type of field - please fill the values fields in the form below; you can also choose if and which field value will be set as default:
You can also set the new filed to have "multiple" values chosen at the same time and to be "required" (the value must be set otherwise the new product won't be added).
After you have saved the custom field, go to Catalog>add new product and click "show field" option to add new field to the form: