If your sending domains already sign with DKIM, then this should not be changed.
SpamExperts will simply forward the DKIM signed messages along to the recipient.
If there is no DKIM signing, you can decide to either sign this on your sending MTA, or sign with SpamExperts.
It's not obliged to sign with DKIM, however it often helps to "authenticate" as much as possible your senders.
Further information on how to setup DKIM with SpamExperts can be found at https://my.spamexperts.com/kb/33/Generate-DKIM-certificate.html