The short answer is that no, this is not a configurable object, especially in an On Demand environment. In an On Premise environment, you can use a VERY unsupported approach with a database trigger, but it is not recommended. If you really, really need this, and you have a fair number of manual transaction entries to do (and if that is the case, you should really be looking into an automated feed), you can create a transaction entry object. That will allow you to make any of the fields required, or with other rules as may be needed. When the user Submits the entry, a process will run to XOG in the transaction entry...and, if you want the additional sophistication, you could even have that submittal go to a designated person(s) for approval before it is entered into the system.
John