You should be able to do that. I have Tradestation 8.7 and I can refer to dlls, although I can't code dll stuff and refer to stuff from other people, so I'm not sure if they're coded in C#.
From Tradestation help:
Here is example sintax to retrieve my balance from Interactive Brokers, using tradebullet.dll:
Code:
DefineDLLFunc: "TradeBullet2.dll", double, "TB2_CashValue", LPSTR;
Bankroll=TB2_CashValue("IB Account number here");