I've never tried to initiate a conference call on Tracfone so I'm not sure if it can be done or not. I don't have the w370, but I will try it later today on my Nokia 2126i and see if it will work. I suspect it won't as I've never seen anything mentioned on Tracfone's website about conference calling so I don't know if it is a supported feature.
As far as being a party on the receiving end, that should be no problem at all provided the party initiating the conference call is using a carrier that supports it.
I also tried a conference call on my w370. ATT is my underlying carrier. I could dial the first number, then after connecting I could dial and connect to the second number....but when I then hit the "link" key I got a "Failed" message. Tried a number of times with the same result each time.