Starting in v3.0 of Stock + Pro you can add the number of shares that you own of a particular stock and see a portfolio summary.
To add shares follow these steps:
1.) Select the stock in the watchlist
2.) Open the inspector pane to the Details Tab
3.) Enter a whole or fractional amount of shares owned in the text field (e.g. 50 or 50.5)
Congratulations, you have now added the number of shares owned!