Override Auto Number field?

Override Auto Number field?

We are preparing to migrate from Salesforce. In Salesforce, we auto-generate a unique number on our Opportunities (Potentials). If the Opportunity results in a contract, we use that unique number as the Contract number.

There are some situations where a contract may be created manually in the field. In this case the contract will have a manually devised number already on it when it comes into the office. In Salesforce, once the auto number is generated on the Opportunity when it is created, the field can later be manually edited to enter the correct number.

However, it appears the auto number field in ZOHO is non-editbale, and I can't find any way to override it. Am I missing something or is this the case? Are there any suggestions for a workaround?