The Premier League's basement side have scored just 17 times in their 20 league games this season, and new manager Marco Silva identified strengthening his attack as the top priority upon his arrival at the KCOM.
Niasse, 26, joins on loan until the end of the season from Everton, where he had failed to settle following his £13.5m arrival a year ago. The deal is believed to include an option to buy over the summer.
Evandro, meanwhile, previously played under Silva at Estoril and joins on a two-and-a-half year contract for an undisclosed fee - thought to be around £500,000 - subject to international clearance.
Both players could go straight into the lineup for the Tigers' home game with Bournemouth tomorrow afternoon.