@kicaduser.. what kind of experience do you got with ALL of this?
I see this essentially is your first topic with us here and I'm starting to doubt you have done anything like this before.
Have you designed a pcb with SMT devices on it yet?
Did you solder it by hand or have it made?
Do you KNOW how reflow soldering works?
What kind of equipment do you got at your disposal?
Soldering iron? What kind?
Magnifying glasses, microscope?
Have you any idea what kind of options exist to mount devices and boards to a carrier board?
You got 2 options here to permanently mount this USB/RS232 converter onto a carrier board to..
1) use the through holes and solder it like a through hole device onto the carrier board, like every sane person would do it.
2) cut/grind/sand the holes to make them castellations and try to use adequate pads on your carrier board and expect problematic mechanical stability down the road. This option comes only up, BECAUSE you don't want to use version 1) - no sane person would do it for 'just experimenting'.
Also, as we're all here to use KiCAD to DESIGN and MAKE our own boards you could copy the layout on that board, get you the devices that you see on it and incorporate it into your own designed board that has everything else on it and solder it with a soldering iron or a reflow oven - no need for 1) or 2) up there.
But this doesn't float your boat either - which means you're stumped and we can't help you - all the while your motivation totally escapes me.
But that's exactly what KiCAD.info is about:
Look at the example for getting from an electronic design to a final assembly made by @ChrisGammell . This is what is meant by using the parts and not the board:
That's it. There are no other sane options out there for what you asked. This is how far you can use the board you got there.
Also, if you got design problems or KiCAD unrelated questions we can surely address them here, but there is websites/fora/mailing lists/etc. which are geared for these kind of questions and have more users online at any moment to address them (=faster response).
Example would be:
KiCAD.info is about support and help around KiCAD software and related things mostly.