What you can do is host the certificate somewhere within your network, either on the pfsense web server or any other internal web server you have. Then you can edit the captive portal page to have a download button for the certificate, and ask users to install it.
However, I don't know how much I recommend using Squid for HTTPS filtering. I'm not having very good luck with it myself, it seems to give all sorts of random problems such as slow browsing, or causing HTTPS websites to not work, certificate errors and all sorts. It seems to really be bodged together, on top of that… It doesn't really have SSL inspection. You're kinda limited to categorical blocking via domains.