It sounds like there'd be less moving parts if you incorporated that existing Server into your Enterprise as either a federated Server or the hosting Server. If there are enough resources on the machine, everything can exist on that one box, otherwise you'll need to look into a distributed environment where Portal and Data Store are on separate machines.
To answer your question, no there shouldn't be a problem federating your existing Server with Portal. Take care to understand how security for existing services will be affected, though, This is described in the Federation section:
There is a tie-in between federation as defined by the IT industry and as used within ArcGIS Enterprise. The tie-in is that once you federate your ArcGIS Server site to an ArcGIS Enterprise portal, the ArcGIS Enterprise identity based security model will take over, meaning any security you had previously set on your ArcGIS Server site will be bypassed by the identity-based security you set within the portal.
Once you have federated an ArcGIS Server site to Portal for ArcGIS, security is handled by the portal; only users who have an identity in your portal will be able to view secured content. Therefore, when you migrate a standalone ArcGIS Server site to ArcGIS Enterprise you will need to recreate your security structure using the ArcGIS Enterprise security model and give everyone who requires access an identity within the portal.
