OK after spending 10 hours today trying to figure it out, I finally did it!
The reason some MIDI files dont work in DTXMANIA is because there is a drumbeat in the first unit of time eg. 1/16th second (there's a musical term for it but I dunno
)
So all you gotta do is delete the first drumbeat and replace it with a "rest" in anvil studio or a similar program
I guess this doesnt really help anyone besides me cos I think i'm the only DTXmania player with tons of midi files