I installed a realtime kernel using pacman but I need to change a thing (I need alsa in kernel space, not as module).
If I change it in the .config file, may I still updating/upgrading the kernel simply using the pacman -Syu command after recompilation?
No you would want to install the locally built linux and linux-headers packages with pacman -U
And you should add linux and linux-headers to IgnorePkg in /etc/pacman.conf. Otherwise your modified kernel will get replaced next time there is an update to it.