For Poloniex:

- Signup there, fund your Poloniex account with BTC.
- Sell your BTC in exchange of USDT (at current rates).
- After this your balance will be keep as USD.
- In the future you should be able to sell your USD in exchange of BTC again (or some other coin).

See: http://www.marketwired.com/press-release/tether-now-integrated-with-poloniex-exchange-1995274.htm

For Uphold:

- Signup, select the "USD Card" provided by Uphold.
- Select "Fund with cryptocurrency", they will generate a deposit address for you.
- You send your BTC, it will be keeped as USD.