Creation of custom codes was introduced in SP20. You can activate other code fields available under Data / Codes / Work Order if you have any left that have not been already activated but if not, you're stuck with the ones you have unless you upgrade.
Of course SP7 to SP20 or higher is a big jump and there are a few requirements to be aware of (attachment storage structure, java version, etc.). If you decide to upgrade, make sure you thoroughly read the Service Pack readme file.
Yes, there is a place where custom codes are activated (Not created. Creation of custom code fields is possible only in SP20 and higher). Go to:
Data > Custom Fields > Work Order
Your Desktop OS code in dev is 1 of 10 custom codes that can be activated for the Work Order entity. You will find other custom fields of data types other than 'Code' (e.g. Text40, Text4000, Boolean, Date, etc.).