If you already have the Footprint and/or 3Dmodel (Step) for the Footprint, then simply place the PinHeader/Socket part you want. Then, Edit the Placed Footprint’s 3D placement (raise it’s Z dimension as needed.
Note: You can also do this for more permanent use by Editing the Library footprint…
Here’s example of mine… which includes SMD pads for Castelation and PinHeader
Symbol would be the same. For 3D model you have two options:
#1 - to edit it yourself (for example in FreeCAD or something similar). There are 3D models of pin headers or pin sockets available online (or even in KiCad 3D models).
#2 - if I’m right you can have more than one 3D model displayed in KiCad for one part. You can add 2.54mm pin headers or pin sockets to the Pico model and offset the Pico itself to sit higher on top of the pins
The files are .WRL and Sketchup files. Can’t do anything to WRL beyond using for images.
I don’t use Sketchup so, can’t comment.
I’m not clear on exactly what you want - looks like you want to use an existing board with Pico and headers already mounted as shown in the repo-WRL.
You can make a Footprint and use that WRL for the 3D. Can raise it’s Z dim (so it’s above the Kicad PCB) when making the footprint. Then, use it - place the Footprint on PCB and place PinHeaders/Socket Footprints underneath it (results would be the same as if you’d used 3D Step file).
Have you looked for pin sockets to buy? You’ll find that they indeed have solder tags that go into THT pads. But the trick is to find a. pin socket that has the right number of positions. Forget about say butting two 20 lengthwise to get 40, or something like that. You’ll find that they will clash in the middle. You must get a 40 position socket connector. I found out the hard way.
You want a mother board (green arrow) with sockets (blue arrows) and you want to solder another two 20 hole sockets in the mother board where the red lines are drawn so you can plug in a Pico H instead of soldering the Pico to the mother board.
That idea is good and it would work. BUT: the mother board project you link does not have holes, it is surface mount, and there is no provision for the outer connectors.
Just a note:
If you plan to solder the pico directly to the PCB with the castellations, then put some spacer in between. Maybe a piece of kapton tape, or just paper will work. (Although paper is not ideal because of moisture absorption) Solder mask is not intended to serve as an isolation barrier, and putting PCB’s directly on top of each other sometimes does create shorts.