If you're referring to something like a xeon phi, then no it won't help. It doesn't work exactly like bolt on power and is designed for heavy multi-threaded multi-core apps for things like programming and parallel computing. The cards run on a version of micro linux embedded and the cores act as a sort of remote computer rather than showing up as additional cores within the os. Not to mention they're quite expensive. It won't help with gaming.
They easily range in price from $500-$3500. You'd be better off with a new cpu and motherboard and upgrading your pc if it's struggling with games. What are your current pc specs? What games are you trying to play?