Yes. Now it is possible.
This is a soft unlock. Meaning you will need to repeat the process every time you change sim cards. You will need a jailbroken iPhone. At this point 5.1 will not work.
So this is what you will need to do.
1. First backup with iTunes. Remove from USB.
2. Then remove ultrasn0w and furiosmod if you have them installed. Remove gevey sim, insert your current sim. (The one that you want to activate). Turn off your iPhone, turn it back on.
3. Fire up Cydia and install SAM from repo.bingner.com. Reload springboard.
4. Now Open SAM, go to More info, if your ActivationState says ‘Activated’ then skip to step 6.
5. Go back to SAM, go to Utilities, Revert Lockdown to Stock, then tap De-Activate iPhone
6. Go back to main menu, Enable SAM if it has not yet been enabled. Select your country, carrier, and make sure to tap on the SIMID
7. Go to more information, scroll down to SAM details, select and copy everything in IMSI, then hit Spoof real sim to sam.
8. Back to main menu, scroll down to IMSI, paste what you copied into it. (overwrite)
9. Fire up iTunes, connect iPhone, it will give you an error but appears to activate, close iTunes, and now Disable Sam (tap enable to disable)
10. Fire up iTunes again, connect iPhone, it will say error can’t be activated, disregard. Close iTunes, fire it up again, and it will give you another error but phone is activated, close iTunes and fire it up again, this time no errors and all is well.
There, now your softbank iPhone is unlocked on your NTT DOCOMO or whatever. Congrats!