Wrong way round WMC. That way would give Inferno priority over SaH. Probably not a problem if SaH has no table files but try it with StL and you're screwed.
You can do this but I see no good reason to do so. You'd make your Freespace 2 folder tidier but the cost you'd pay is that you'd have to type in the name of the mod manually or spend a little longer browsing to find the sub mod. Putting a mod folder within another mod folder doesn't automatically give the mod access to all the resources of the other mod AFAIK and even if it does the resources of the main mod would take priority most likely (Meaning that any tables in say SaH would be overridden by those from Inferno).
What you really want to do is to set up a mod.ini file. You can read some background on them
here and also grab some examples which work for SaH and Inferno.
If you still want to put the SaH folder inside the Inferno one it should work simply by dragging the SaH folder into Inferno. Don't know why you'd want to do it but doing so would give you what you asked for.
Any questions, just ask. I'm trying to pimp mod.ini files cause I think more people should be using them
