The trade exception can be used now.
However, it can't be used to do what you're trying to do. Basically, it can be used to absorb the entirety of a player's salary, without having to send anything out in return. So, we could trade for a guy like Lamb, because his entire salary this year fits within our exception. We couldn't trade for Kobe using the exception, because his salary doesn't fit within the exception.
Also, while it's technically possible to use the exception now, it's unlikely we will unless we clear some other salary somehow. We're right on the cusp of the luxury tax, and ownership is determined not to pay that this year.