We bought legal protection as an add-on to our home insurance policy.我们购买了法律保障险，作为家庭财产保单的补充。A printer is a useful add-on.打印机是个很有用的电脑附加设备。There are no add-on costs.没有额外费用。They will sell a voice recognition product as an add-on, on-demand service for their users.他们会将一个语音识别系统作为附加的随选服务提供给用户。