Let me tell you that I have not tested my following advice, so the steps I tell you to do might or not work. I believe they do work, but you need to test it. 
Thank you so much for your answer. I asked in this forum because I believe p4 switch may impose some kind of limitations.
I think if there are any limitations, it is about Mininet probably. But we can sometimes try to help, of course 
A link is shown between root-eth0 and s1-eth5. However, when listing mininet networks’ nodes I cannot see any new host. I would like to place one host in root namespace, open its terminal, e.g. xterm h5 and then, as it is in the rootnamespace, would like to test I can ping to my host ip address(outside mininet). My aim is to be capable of accessing to some services running in my host machine from a mininet host. That’s why I believe placing a host in rootnamespace would make it possible.
So, you probably do not need any “host” since you already have the actual “host”. I mean why not use the terminal from the “host machine”?. You will need additional “route” commands and ARP rules in the host machine to be able to route packets out of root-eth0 AND rules in the switch to route packets towards root-eth0. Not sure if you understand what I mean. Once you enable forwarding between Mininet and host machine, Mininet hosts should be able to access services running in the host machine.
I made a “kinda” descriptive picture:
Hope it helps Sally.
Cheers,