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]
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Dictionary attack - Thread 4 - Password found: abc1234
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it may take a while but especially if the password was weak, then your chances are actually pretty good