I wouldn't recommend it... There were changes in the backend database between 2018 and 2017.
We recommend to run the same version across all products (SOLIDWORKS, SOLIDWORKS PCB and SOLIDWORKS Services).
However, if you need to use SOLIDWORKS PCB 2018 with an older version of SOLIDWORKS CAD, you need to download the appropriate hot fix if you are not using the same version.
As example, SW PCB 2018 can work with SWx 2017 SP5 with the HotFix_1041422
Territory Technical Manager, SOLIDWORKS PCB