The way I found for El Capitan:
On the Source Mac:
1) Open Keychain Access
2) In Local Items find the "Signature Annotation Privacy"
3) Right-click the item and select "Copy password to Clipboard"
4) Open TextEdit and paste the password (note the password is long) and save the file as text
5) Transfer this file to the Destination Mac.
On the Destination Mac:
1) Open any PDF in the Preview
2) Create a dummy signature using your trackpad and save it
3) Close Preview
4) Open Keychain Access
5) In Local Items find the "Signature Annotation Privacy" and double-click to open the item
6) Click on "Show password"
7) In the password field paste the text from the text file you created on the source Mac
8) Click on Save changes
9) Close Keychain Access
The signature will show up in place of the dummy signature in Preview.
Hope it helps