Is it possible to convert a master .esm file into a plugin .esp file?

What about vice versa?
If that's not possible, how do I convert a .esp to a .esm?

Wrye Smash > select file > R-Click > Copy to esp/ESM

You can ESM'ify with the CK by changing the file extension from esp to ESM, then opening as active and saving.

Also TES4Gekco can do it, actually it's just a matter of changin a single byte from 00 to 01 and renaming from .esp to .esm.

Technically, there's a bit more to it than that since the advent of http://i.imgur.com/VaIrx.png. Any ESM flagged plugin overriding its masters' cell children, regardless of its file extension, should have an ONAM list in its header listing every overridden ACHR, LAND, NAVM, PGRE, PHZD, and REFR (if applicable). Version control, evidently, can create ONAM lists as can *Edit.

Nothing that complicated, I'm just talking about the simple conversion, specifically what TES4Gecko does when you hit the "Convert to Master" button. Merging and binding plugins are beyond my reach, until TES5Gecko is released.