My Haft was atacked by some strong npcs and they´ve cut the arm of one of my survivors, and now its crippled. And now? We can craft or find mechanical arms? Or i´ll need to kill my survivor?

Hello.

Once you have some artificial limbs, Select the crippled one, check the "health" tab, click on any crippled part, select a new limb for him/her.

Artificial limbs could be obtained by crafting on metal work bench or traders.