I think this should work if you set driver = alsa_usb and device = hw:Extigy in lirc_options.conf (copy /etc/lirc/lirc_options.conf to /storage/.config/ and edit these two lines).
See the wiki for more info Infrared Remotes [LibreELEC.wiki]
You'll probably also need to create a /storage/.config/lircd.conf file matching your remote. However I'm not quite sure which remote (protocols) the Soundblaster receiver actually supports - from a quick glance it seems IR decoding is performed by the soundblaster hardware so it could well be that the IR receiver only works with the remote that shipped with it.
It's probably easier if you keep your MCE USB receiver (that one will work fine OOTB) and maybe use a USB hub if you're running short on USB ports.
so long,
Hias