Yes, you can import a certificate (which is different than a signature) the was/is used elsewhere. In Acrobat 11 you can find the .p12 file by selecting: Acrobat > Preferences > Signatures > Identities & Trusted Certificates > More
and in the left hand panel: Digital IDs > Digital ID Files
and select the one you're interested in and select: Certificate Details > Details > Private Key
and you'll see the path to the (.p12) file.
The same dialog allows you to add a new one.