I have that same spoiler extension and I set it in place with the OEM spoiler installed on the car to check fitment (ended up needing to sand out a good bit of filler from the middle of the mounting pad to get it to sit properly), then took the OEM spoiler off the car, measured and marked both parts on my workbench to make sure the two spoilers were aligned properly, drilled the OEM spoiler and installed rivnut inserts, then liberally applied VHB tape to the mounting surface of the Sequence spoiler (after cleaning everything with 40% IPA) and dropped the OEM spoiler into place, heated everything liberally, clamped the parts together, and fastened into the rivnuts (I forget the length of the fasteners I used); then after 24 hours took the clamps off and remounted both spoilers as one full assembly onto the car.
Definitely would have been easier with 2 people, but it worked and it's held up perfectly for well over a year now. If you're not into rivnuts, self-tappers are a viable solution as well, just as long as you're using good 3M VHB adhesive tape between the two parts and you clamp the two parts together to let the adhesive set up before you take off down the road. I definitely would not trust VHB alone -or self-tappers alone- to hold that piece onto the OEM spoiler at speed - the combination of the two, and proper cure time for the 3M adhesive, is critical.