@MoralesAFD you have now devices enrolled to MDM (is this Meraki?) and you want to enroll them to the DEP but don't want anything to happen to the devices?
Device ALPHA is in MDM but not in DEP?
You want to add device ALPHA in DEP but don't want to re-enroll it?
If my assumption is correct, then yes this is possible;
1. You can leave this device in DEP and not assigned to any MDM. Nothing will happen to the device
2. You can assign this device to a MDM but without pushing any DEP setting to this device. Nothing will happen to the device
3. You can assign this device to a MDM and assign DEP settings to it. At first nothing will happen to the device. This will only be applied to the device on factory reset, resulting in device enrolling in MDM through DEP.