I think you can just use CONN_01X00 pin header parts to represent tubes on the circuit schematics, then define just the footprint for the tube, either from its datasheet or ANSI/ISO/DIN/GOST connector specification. Then that footprint can be assigned to corresponding J0 on the imported PCB file. You can also print the PCB to visually test fitment. That might not be the correctest way but nor should be the wrongest approach.

One of the most confusing things when I started using KiCad is its two main features, EESchema and PCBnew, are basically two independent open source projects. So they're not tightly coupled, but works by importing and exporting files and manually assigning items in one side to the other.