1st of all this is my rig:
I5 4670k 3.4 GHz
8g ddr3 1866
Rx 580 sapphire nitro
power supply seasonic M12 II 850 w bronze
i'm planning to add my old r9 280x to this rig for mining purposes (using riser) can the PSU capable of supporting the whole rig (2 GPUs)
A 280x is going to lose you money since it consumes more money in power compared to coins it will mine. Your overall rig is just inefficient to mine on and will likely just be a break even at the end of the month.