This is quick tutorial that I made to reply a post in psx-scene (it may be incomplete, but it should give you an idea):
0-Run PARAM.SFO Edit.exe as Administrator to associate .SFO/.SFX extensions. Close it.
1-Backup your save data folder to a USB storage device (/PS3/SAVEDATA) or download them to the PC from /dev_hdd/home/0000000x/savedata via FTP
2-Backup your savedata folder in a ZIP/RAR/7z (just in case you make a mistak and corrupt your saves)
3-Start the Bruteforce and browse to your local savedata folder
4-Browse to a savedata with your account id and "Use PARAM.SFO as template" (you can do this manually copying the PARAM.SFO as "template.sfo" into the folder of Bruteforce)
5-Browse to a savedata with a foreign account id.
6-If the game has a key in the database it will be displayed, otherwise you will need to bruteforce it double clicking on the game and providing the ELF of the game (EBOOT or another self)
7-Once you have the key for the savedata to be resigned, press "Update Account ID" to update the PARAM.SFO with the account id in the template.
8-Press "Patch SFO" if the game is copy protected (it says YES in the Protected column)
9-Select all the files from the PFD using Ctrl+A and press "Decrypt PFD"
10-Use "Update PFD (Partial)" or "Update PFD (Partial update without Game setting) to update the hashes in the PFD. If you have the disc_hash_key for the game, you can use "Updae PFD (Full)".
11-Once the PFD is updated, select all files usingCtrl+A and press "Encrypt PFD"
12-Press "Verify PFD" to check if everything is OK.
13-Copy the resigned savedata to a USB storage device (/PS3/SAVEDATA) or transfer them from the PC to /dev_hdd/home/0000000x/savedata via FTP
