It is relatively simple to take HaloCE content and put it into Xbox maps.
By "content" I mean just about whatever you want. i.e. both existing and newly created tags
By "relatively simple" I mean that it's only slightly more difficult than taking content from one Xbox map & putting it in another Xbox map.
Disclaimer
Obviously, not all content will be equally difficult to transfer.
i.e. moving over models/bitmaps/sounds will be harder than moving over collision models/physics & the like.
Also considering the differences between certain HaloCE & Xbox meta,
& the differences in the format of the map file, there may be a small learning curve.
Also, the following assumes significant knowledge of modding Halo using several existing tools.
For more info regarding the HEK, please refer to the tutorial that came with it.
Anyway, here's what you do:
Tools
The usual: Tool, Guerilla, HHT, Hex Editor, 3dsmax or gmax
(Sapien isn't necessary, so if Sapien doesn't work for you, like it doesn't for me, then YAY! for you.)
1.) Use 3dsmax or gmax to create a basic Halo level.
Follow the HEK tutorial but only go through "Level Creation - Part 1" & skip the smoothing groups.
Also skip "Level Creation - Party 2" as it's not necessary for this.
Now export the jms & compile into tags.
EDIT: You can skip this first step if you want and just use the tutorial level as your map. It really doesn't matter at all. You may want to make a backup of the original files.
2.) Open the new scenario tag in Guerilla & add whatever you'd like.
Hell, add everything if you want. heh
EDIT: If you are using the tutorial level, then step 2 here is where you'd be opening your tutorial.scenario in Guerilla...which should be obvious.
3.) Compile into a .map
The tags that come with the HEK are significantly different in their "raw" form than they are after they are compiled into a map.
In fact, many of the tags, once compiled, are identical to their Xbox counterparts. See? That's the key.

SO, this dummy map is the map you will use to add content to so that you can then compile it and thus, "convert" the tags to a form that will easily work in Xbox maps.
You will not be playing this map, so it's not necessary to put a lot of effort into it.
EDIT: If you are using the tutorial level, you may want to make a backup of the precompiled tutorial.map
4.) Open the map in a hex editor, go 0x4 bytes into the file and change 6102 to 0700.
This is the map version number....must be the HPC version number of 7 to open in HHT/HMT.
5.) Open the map in HHT, find the content you want & extract the meta.
6.) You can now inject this meta into an Xbox map.
This will be slightly more or less work depending on whether you are adding a new tag to the map or updating an existing tag.
If you are updating an existing tag the updated meta size may or may not be the same exact size as the existing meta, depending on what changes you've made.
If it's the same size, then obviously you can just inject the meta right over the old meta....super easy.
This is usually the case if you've only updated something similar to what the HMT plugins modify. If you add a function or something then the meta will probably end up larger.
If the updated meta is larger now, or you are adding a new tag then you'll of course need to find something to overwrite....still pretty easy.
7.) Now you need to adjust the dependencies in HHT as they'll likely be completely wrong.
See? It's almost exactly the same as moving content between Xbox maps.

This way you can make all kinds of changes to existing Xbox content that we could never do before, or to create brand new Xbox content using the HEK!
Just don't make more than 2 scenery objects spew blood...too much lag. hehe
Obviously it would be much more convenient to be able to directly convert the H:CE tags to Xbox injectable meta,
but this pretty much has the same outcome.
Until we have some sort of tool (no pun intended) to compile Xbox maps, we have to do things like this.

I hope someone finds this useful.
