No. The 7100i uses the iDen network and is not compatible with the T-Mobile network. It is not a software/unlock/whatever issue. It simply can not be done. The 7100i can only be used on iDen network like Nextel. It can not be used on T-Mobile.
Forget the words SIM and unlock for now.
You can not use the 7100i on ANY OTHER NETWORK besides Nextel (and their associates), which does not include T-Mobile.
The phone can not be used on T-Mobile.
You can not modify the phone to use it on T-Mobile.