Hmmm. The easiest option, whilst not the most realistic, would be to rename Suvarnabhumi to use the VTBD ICAO code, that way flightplans recognise that the Suvarnabhumi scenery is where the plane is being guided to go due to its code.
The optional, more realistic, yet harder method is do de-compile all of your flightplans and open up the flightplans.txt folder and using notepad use the Replace, Replace With search and replace VTBD with VTBS. The you would need to open up airports.txt and add a VTBS line, along with the co-ordinates of the airport. Depending on how many flightplan.bgl files you have, this can be a long process, unless you compile all of your traffic into one bgl (Which is what I chose to do with FlyTampa Kai Tak).
Hope this helps,
Cheeks