I mention how to do that in this post., though its not a well made description of how to do that. Simply explained:

• Convert your coins in coinpot to 1 specific currency of your choice, for example LTC.
• Withdraw that to your personal wallet for that coin, in this case a Litecoin wallet.
• Go to BlockTrades(through your Steemit wallet), then select your coin, and send the coins from your wallet to the address listed on Blocktrades.

This is what i do with Coinpot to get Steem Power, though I use Dogecoin instead of litecoin.
Hope this helps!