Is it possible to built the route using Lat/Long on A320 FMS?
Well, in the real thing you can!
I am currently cruelly separated from my Sim PC and in a hotel on business with only my laptop so can’t try it out. You - will have to be your own Tester!
Here’s how you’d go about it real world;
In the Legs page you would enter a Waypoint with a name you’ve given it yourself (such as CUST1) and the MCDU then prompts you for the LAT/LONG which you can input in either 7 or 11 GPSS Character Format. Then you can do a DIR as usual - whatever you inputted it for - it’s just another Waypoint.
Not sure if that’s implemented in either Default or by FBW.
Give it a shot!
Let us know how you get on.
On Both, FBW and Default unable to create and store waypoints like on the real thing.