Yes, you can add as many MDMs as you want to Apple Business Manager. In the ABM I was managing, I had Jamf Pro, a Jamf Now trial and one or two other test MDM products all in there as valid MDMs that devices could be assigned to. While in Apple Business Manager, click on Settings in the lower left sidebar, then click Device Management Settings and you should see a button called Add MDM Server there, like this:
You'll have to have the public key from the MDM (Jamf Pro) handy to upload so it can generate a token to upload back into Jamf Pro, which adds it as an MDM server that can have devices assigned to it.
The key thing to know is that devices will get assigned to only one of them, which is up to you. If you set the auto assign option, then new Macs or iOS devices can be automatically assigned to one of the MDMs as they enter Apple Business Manager. Or you can assign them manually.
But if you wanted to assign all new Macs going forward beyond a certain date to your Jamf Pro instance, then that will work to direct them to Jamf Pro instead of Jamf Now. You can of course re-assign any existing devices to Jamf Pro inside ABM, but they won't get enrolled into the new console without either wiping them and re-enrolling or going through some sort of intervention, like a user initiated re-enrollment on the Macs for example.
