Hi Teriod,
You can add an additional platform at any time by making use of the PokerStrategy FDB (First Deposit Bonuses) and depositing your own funds. As long as you use the PokerStrategy download links and marketing codes, your account will be tracked and you will receive PokerStrategy points on each platform used.
To actually transfer your existing funds from partypoker, you would first need to clear the starting capital (see link below) and meet any withdrawal conditions that Partypoker themselves set, e.g. ID documentation, points total etc.
It's usually quite straight forward, most members (myself included) usually have at least 2 or 3 different platforms linked as it's good to swap and change now and again.
Good luck, let us know how you get on.
Best regards,
Bart