As Amol writes using G/L Account is possible and a very easy way to do it, as your users simply select a designated G/L account and then writes the text. But that would require you to allow you users access to the see G/L accounts, which many companies don’t want. Also you would get no statistics on qty. sold or have no predefined standard prices.
But if you want functionality similar to the service items in GP, then you might want to consider using the Resources in NAV. A Resource is normally a named either person or machine, but can really be any service you’re selling. And you can setup prices for different services and get statistics. So if you want to use the “service items” on your sales orders/invoices, then this might be a better option than using G/L Accounts.
The disadvantages of the resources is that you can not use them on Purchase Orders/Invoices, only on Sales Orders/Invoices.