That sounds very familiar.
This topic crops up from time to time and to my knowledge there has been no definitive solution given.
https://forum.solidworks.com/message/160343#160343 is a typical thread. A more thorough search may reveal others more helpful.
If it's anything like a regular time sheet for job costing, you shouldn't have to be too granular in your reporting. If some parts aren't assigned an order #, they should at least have a product or customer name which you should be able to use.
For the reasons you described, don't even try to assign times per individual part or drawing. Unless you have really anal &/or suspicious managers, an overview of time spent daily is usually sufficient ... 3hrs on Job 123, 2hrs for customer XYZ, 2hrs admin tasks, and don't forget the 1/2 hr per day filling in this stupid form.