You can do it that way but I wouldn't.
If you put the m3u in "Settings\Source\Channel Mappings\Import m3u" it has to exactly match up with your xmltv file.
It also won't let you add in other addons very easily as it gets reset on every restart.
I'd either add it in as an "addons.ini m3u Playlist subscription" or just add it in "Menu\Stream Setup\Import Playlist".
The addons.ini method will do fuzzy channel name matching with your xmltv file and is much more forgiving.
But experiment. It might be just what you need.
Back up your addon_data\script.tvguide.fullscreen folder and try all the ways of doing it.
There are some output text files there that show you what stream is mapped to what channel.
The heart of the channel mapping is the addons.ini file. So look at what is created there.
Use this to look at the tables in the source.db file.
http://sqlitebrowser.org/
You'll be able to see how the channels from the xmltv file map to your streams in your m3u file.