Maxim Said: It's easy to do.
1. Load the ROM in a hex editor
2. Find the level names (0x4548b)
3. Copy/paste and number them in hex from 0
4. Write the number sequence for the start of time trial mode:
BBQ Burnoff = 02
Science Lab = 40
Hammer Havoc = 33
5. Search the ROM for those bytes (0x452c5)
6. Type the values from your list to replace the Time Trial tracks - remember to back up the original though. Don't type too many, it'll break the ROM - there's room for 27.
Some of the levels are likely to be broken, eg. the ones called OLDTRACK.
You can use the same numbers in a "value" Action Replay code search which can help when you want to trigger certain tracks (eg. bonus games) in modes other than Time Trial. (The bonus tracks have no circuit defined so they bug out in Time Trial mode.)
I found the level names and edited "Driller Killer" to "Toolbox Terror" just to test it. But firstly, what do you mean by "0x4548b"? And, where do you get the number sequence for the start of time trial mode? Also, how do you search for things like 0x4548b and 0x452c5? I'm completely new to this BTW. <_<