Yes. You can have trades open on multiple accounts/trading platforms (e.g ActivTrader, MT4, MT5), while still being able to move funds in and out of your main ActivTrades account.
This is because your open positions are secured by the balance in that individual trading account (which you cannot make withdrawals from).
In your Personal Area you can move funds between all of your trading accounts at your own discretion and can easily withdraw whatever is not being used to secure open positions.
Related Articles
How Can I Start Using ActivTrader?
How can I see my trade history in MT4/MT5?
Can I use MetaTrader on a Mac?