OK, here what I found out.
1. At 710AD, I got nationalism (Can u believe that Rome and France still in ancient time ?)
2. I made sure that I still could sell contact.
3. At 720AD MPP with Rome (which he gladly accepted of course
)
4. Still at 720AD, declared war to France, and placed an unguard worker on the border, so He would happily attack my worker.
5. At 730AD, France attacked my worker, which triggered my MPP with Roman.
6. I checked the status, and I COULDNT SELL THE CONTACT
By the time roman declared war to france, the system automatically introduce France to him.
7. Stop the test
Well, everybody was right. Its impossible in the standard game, only a mod with special utility (such as multitool). However, by using multitool, CivIII vanilla could make some kind of "locked MPP." like in Conquest (IMO, its better than conquest, if we mod the game well, we couldnt break enemy's locked MPP)