Technically yes. However, the answer to your question is a bit more complex than a simple yes. There are devices in your fabric that you would use to fuse together your so called legacy network to your SDA fabric. Let's say you have a part of your legacy network that needs to traverse your SDA fabric to get outside or beyond your fabric. You would have an internal border node (IBN) that essentially is a ibgp peer to your external border nodes (EBNs) and an ebgp peer to your legacy gear. These types of devices are essentially devices that are used to connect to other parts of your internal network/s. Lastly, you could manage your legacy equipment on the same underlay if that is what you wish via an IBN for example. Another SDA node role you should be aware of are the fusion routers which act as your default way out of the fabric which are external facing. These types of devices are ebgp peers with your EBNs. These also allow you to fuse (route leak) between your SDA VNs. I suggest taking a peek at the following resources to gather additional info:
https://community.cisco.com/t5/networking-documents/sd-access-resources/ta-p/3812030
HTH!
