Probably, but there are two separate issues with this.
First, you can add an additional auxiliary at anytime anywhere if it’s using the virtual N-way method. Regardless of whether there are other wired auxiliaries or not. Since it has its own radio and it talks to the smartthings hub directly, it doesn’t matter what else is talking to the master. So that part is good.
The second issue has to do with the specific wiring at that particular location. Your existing wiring might require that a device at that location be wired into the circuit loop and then it can get a little more complicated. If you were going to make all The auxiliaries virtual, it’s easier in the sense that you just don’t use any of the traveler wires, make the master independent, and then wire each auxiliary separately. But if you going to loop some of them it can get complicated. So it will just depend on the actual wiring. But in general, providing you have the right device model at each location, you should certainly be able to have some auxiliaries which are using traveler wires and some which are communicating directly to the hub instead. People often use this to add an additional auxiliary switch in a different room on an entirely different circuit, for example.
Is the virtual association in smartthings accomplished in smart lighting?
That’s one way to do it. You just have One switch “follow” the other so that when switch A comes on, switch B comes on. A lot of people set up a mirror so that it’s also true that when switch B comes on switch A comes on, so the statuses stay in sync no matter which which you physically press. (This mirror doesn’t usually create an endless loop, it depends on the exact model of device being used. But typically if the second device is already on, then the command is not sent so the loop stops.)
Do I need something like Core?
No, although you can do it with core or webcore if you want to. 
