The first is to get copies of the public and private key that are in use by the other service and provide that to our Support team. We will partner with our Deliverability team to work to process the request to have these keys copied into Marketo, so that both services use the same key.
The second option is to have a custom DKIM selector implemented. For example, there is already a DKIM key set up on m1._domainkey.unicorn.com. We can set up a key for m2._domainkey.unicorn.com or another custom value in place of m1. Let Support know what value you'd like to use for your custom selector and we will partner with our Deliverability team to implement the request. Afterwards, you'll be able to set up DKIM as detailed in documentation: https://docs.marketo.com/display/public/DOCS/Set+up+a+Custom+DKIM+Signature
Once the request has been submitted, please allow 3-5 business days for Marketo to process the request. Marketo's Support team will contact you when the request has been completed.