[otrs] Time units, normal time vs. overtime

Hi, I'd like to "expand" the "Time Units" field when handling tickets. My users have a need to distinguish between normal time used on a ticket and overtime used. (for billing and statistical purposes, for example) I'm imagining that it's a fairly simple thing to do, but i have no idea how to, as i'm really not a coder. Can anyone help? Have also had a look at the "Time Accounting" module, but... I just didn't get it. And it seems a bit overkill for what i need. Thanks. -- /Sune