It can be, but isn't necessary. Bethesda fixed the navmesh bug with Patch 1.6 and CK 1.6.89. So you don't need to flag yourself as a false ESM in order to get NPCs to behave. Which means if you want to upload to Steam, you won't have any issues in leaving it as a normal ESP.
they did, really? how disappointing - just like hearing someone finally caught the loch ness monster or something - there goes a legend...

but apart from that: so there's no more navmesh bug. and i've read esp's could now somehow detect eachother without any further trickery (via form lists or whatever, yet gotta dig into this a bit more) -
so what further reason would there still be for an esm anyhow?
...ah yes common masters for shared items, i don't think this goes with esp's now, or does it?