No, unfortunately that's not the way TQ mods work. The simplest mods, like xmax, overide configuration from the database (when you load the map) of the engine.
Making an xmax without x3 bosses is possible, but I am not interested in making it. I'm real busy with real life.
If you want to make it, I can tell you the changes to what database tables you would have to make.

Yes please. I could then figure out the rest.

Ok, first you'll need to learn how to mod. That should be fairly easy, since you only need to use the Art Manager to do what you want.

Here a thing to get you started with setting up the Art Manager properly:
http://www.titanquest.net/tq-forum/threads/31156-HOWTO-Setup-Artmanager
Read "How to install the templates to open dbr files", "Where to set Artmanager directories to" and "MultiSelecting Database Files".

So the tables you'll need to edit are /records/xpack/game/gameengine.dbr and /records/proxies boss/pools/boss/ALL OF THEM!.
To make the mod xmax, set spawnMaxModifier and spawnMinModifier in gameengine.dbr to 1000. I think that's in the proxy section in Art Manager.

To only spawn a single boss, you're going to have to open a bosspool, and delete two entries in the pool section eg name1, weight1 and name3, weight3. Leave only name2, weight2.
That's where "MultiSelecting Database Files" comes in handy, you can do most of the bosses at the same time.

To mass import, read this thread to extract the archive with ARZextractor (http://www.titanquest.net/tq-forum/threads/55679-How-t-extract-database-arz-without-the-vault) and then copy the .dbr files manually to the working directory of the database.
That ought to save you time instead of importing one by one.

Oh, and Immortal Throne bosses are at /records/xpack/quests/proxies/pools/