you can’t add a .p12 cert into your keychain if you dont have the password. but you can try to brute force it with crackpkcs12 you’ll need to compile it as instructed, then download a dictionary like this.

$ crackpkcs12 -b -d crackstation-human-only.txt -v Certificates.p12

Dictionary attack - Starting 4 threads

Performance:             41072594 passwords [  173037 passwords per second]
Dictionary attack - Thread 4 - Password found: abc1234

it may take a while but especially if the password was weak, then your chances are actually pretty good