SPF, DKIM, and other similar methods are not compulsory for mail to be accepted and delivered. Services or people that force senders to implement such techniques are making a mistake (or they're simply ignorant about the standards).
Up until now Domino does not support DKIM (nor SPF check for receiving). I've read it in some SPR that Domino's development plan doesn't include these enhancements in the foreseen future.
My suggestion would be to have someone develop a Domino plug-in for you to achieve the required functionality (one-time payment), or route your mail through a 3rd party service that offers such features (monthly or annual payment).
But always remember that the implementation of SPF or DKIM is not compulsory in the standard, and try to educate the mistaken ones about it.