I understand that I can add the cost of acquiring a stock to my cost basis as reported on a 1099-B. How do I do this using turbo tax?

You will enter the information in the federal interview section. 

1. Select Income & Expenses 
2. Scroll down through all income until you see Investment income 
3. Select Stocks, Mutual Funds, Bonds, Other 
4. Select Add more sales or Edit if you have already entered sales information
5. Continue through the screens until you see a screen titled Now, we'll enter one sale on your ....1099-B.
6. You will be able to scroll down and enter your cost basis, date of acquisition, sales dates, proceeds, etc. on this screen. 
7. Make sure you do this for all sales listed on your 1099-B to ensure you are only paying tax on the gain for each sale

you can't add the cost to your cost basis.

your cost of acquiring is the cost basis.

If you have commissions, your statement already includes that amount.