Yes. But why would you need to? In the positive-sum trade scenario, you're gaining from the trade, so precommitting to accept it is unnecessary.

If you mean that I could precommit to only accept extremely favourable terms; well if I do that, they'll choose someone else to trade with; just as the threatener would choose someone else to threaten

Them choosing to trade with someone else is bad for me. The threatener choosing someone else to threaten is good for me.

/\ That is, in many ways, the most important distinction between the scenarios. I want the threatener to pick someone else. I want the trader to pick me.