I've looked and maybe overlooked, but I want to add new hardware to existing ESXi host.
Meaning, I have working ESXi host 7.0.3c and I want to add physical NIC card (4 ports NIC).
Will this work and will existing ESXi host "detect" and add new hardware during boot or this is some sort of taboo for ESXi?
First you need to check if the card is compatible with ESXi. You can do it from the reseller site or from the vmware compatibility matrix site:
Normally the drivers are already included and on reboot you should see the new cards. Otherwise, specific VIBs have to be installed (it happened to me a few times and you can find all the documentation on the website of the manufacturer of the card you add)