You can just install those kits into your lights....but there are several issues with HID kits.
1) First and foremost most of these kits are not reliable and tend to be of very low quality. You might get a few months out of them......some even die within days. Even The Retrofit Source (TRS, a trusted hid parts company) has had major issues with their kits (ballasts mostly). Some kits will work but bulbs won't match each other, have awful output/alignment, or have flickering issues.
2) In any case if you try to put an HID kit in stock halogen reflectors, there will be annoying and unsafe glare experienced by other drivers around you. Your output will also be spotty and have excessive foreground illumination, restricting your ability to see properly further in the distance.
3) Depending on the HID bulb and the headlight, there may be damage caused to your reflectors by installing said kit...e.g. haze, burns, melting (more common in fog lamps with kits).
Now the absolute best option for an HID conversion is to retrofit your stock lamps with HID projectors, and for the best reliability, use all OEM parts (save shrouds and maybe relay harness). This, however, is a more costly option, especially if you cannot do it yourself. There are several companies that will perform the service for you, some cheaply, but choose wisely. You get what you pay for in the lighting world, expect to shell out $400-500 in parts alone. If you'd like to know a bit more, feel free to PM .me.