The existing Answers are good, I just wanted to add something on top of them.
As you're doing Scrumban, I am assuming/hoping that you are performing the Scrum event known as the Retrospective (often considered to be the single most important part of Scrum).
Therefore, whatever you take away from this, other research, and your own internal musings, I suggest ...
I would like to challenge your assumption that this has anything to do with the WIP-Limit.
Lets say you have 5 full time developers and 2 part time developers. And for starters, you set the WIP limit to 7, so that each can concentrate fully on their one task until it's done.
Whether or not the part timers are there on any given day does not change ...
I believe there's a simple solution: All Part timers must pair program with full-time developers...all the time. But that seems heavy handed?
I have worked with teams that pair on all work and it has been highly effective.
The developers will need to think carefully about how they pair to ensure they maximise the benefits of this approach.
There are lots ...