If it is for information display you could use settings You can create a new setting called Product.CustomFieldName.X where X is the product Id Then to access the field for say product Id 20 then get the value for the Setting Product.CustomerFieldName.20
string CustomFieldName = string.Format("Product.CustomFieldName.{0}", productId); int CustomFieldValue = _settingService.GetSetting(CustomFieldName, ...
Otherwise if it is to store a customer value use Product Attributes
Could you provide further details for what the fields will be used for?
Keep in mind if you use the approach offered by Yidna, you want to keep your settings clean, insure to delete values when deleting products but this may lead to loss of information. You'll need to consider whether the information may be needed for future reference.