You don't want to make it stronger to impact. You can make it more stable by weight training, but what exactly would impact conditioning be improving? Toughening the cartledge so that the knee cap can't move as much? Building up scar tissue? Do you really want that for the knee?
With the knee being so strong, there is a good chance that you can bust someone's shin with it, but if their shin hits at any sort of angle, or even straight on an skips, or maybe even just straight on, and you get some sort of movement of the knee cap, you are going to be really unhappy.
About a year ago, a fighter / teacher I know got messed up. He was one of those guys pro fighters run from. 235 pounds. Thumb pushups. Black belt level Judo, TKD and years and years of Muay Thai. He could jump in the air and triple kick. Just crazy. I've seen him corkscrew jump in between a full speed spinning hook kick and land in a scissor take down. I've heard about him man handling a BJJ black belt. The guy is just on a whole-nother level. I can't emphasize that enough.
He got put in front of a new guy at a gym who had been hurting people, but no one warned him. This guy hauls off with a full power round house to the leg. The super fighter points his knee at it, the way he always checks kicks, in my opinion under the false security of light sparring where pain compliance works well, and ran into the actual mechanical issue.
The new guy busts his shin, hits the ground almost crying, can't walk, has to be driven home, quits, no one sees him again. Probably he fractured his shin.
The established guy finishes sparring, goes home, his knee swells up, and is forced to go to the hospital. He ripped some ligaments or something and had to rehabilitate his knee for 6 months.