So i was going to try to final coat my cake with spreadable ganache - but i cant get all the bumps out
can i add this ganache to butter cream to make a more spreadable icing?

bumps as in unmelted chocolate? if that's the case it will not get smooth even in buttervream. You may try remelting it slowly and go over it again. Otherwise I have seen a youtube video where she used a metal spatula dipped in hot water to melt/smooth the bumps out, but the warning is don't get too much water on the chocolate, itself that will make a bigger mess. Just use enough to warm up the spatula and smooth it out.
Hope that helps.

Was your ganache cold? To get it super smooth you would want a room temp/luke warmish ganache, it will set up once it's on the cake, but will be warm enough to smooth it out. Good luck. Otherwise I would try a heated spat to smooth it out. I worked somewhere where we had a hair dryer just for heating up a knife for cutting through frozen cakes and cheesecakes. Good luck!

If you got any water in it , it more than likely seized on you.

Try Microwaving it for just a few seconds. Since I work with cold cakes the ganache will get cold as a spread it on. Then as a work to smooth the crumb coat and I start scraping the extra ganache off and putting it back in the bowl it leaves a cold lump of ganache in my bowl. Therefore, before I put the final coat on I warm the bowl up a little to get the smooth texture back.