Can I convert my XC bike into a trail bike?

Only to a limited extent. You can install a longer-travel fork (check frame manufacturer's maximum), wider tires (if frame clearance allows), and 4-piston brakes. However, the frame geometry—head angle, reach, chainstay length—cannot be changed, and these are the primary factors that differentiate XC from trail bikes. A 120mm XC bike with upgraded components is more capable than stock but still won't match a proper trail bike's descending performance. If you consistently want more capability, it's better to sell the XC bike and buy a trail bike.