It is certainly possible to do what you ask, you need to make certain you are exporting Assignments and to do it time phased. Users than enter time phased values to map back to MSP.
A word of caution, this is not a simple map back. You need to consider time phasing (and the periods involved). In addition to the time periods involved, you need to respect "today" and the "status date" so you don’t automatically load assignments in the past.
As it turns out, i am just finishing a project extremely similar to what you are talking about and would be happy to demo it for you to give you some ideas. Can’t give you the code, but I can show you how it works so you can implement your own solution.
I just completed a final peer review on it and have about 20 actions to work off. That said, it is still plenty viable as a demo for you.
In closing, think of MS Project as a database with three tables: Tasks, Assignments, Resources. Each have their own primary key. Of course there are many more tables, but this is where you need to focus your attention.
jeaksel at yahoo dot com
Jim Aksel, PMP, PMI-SP
Microsof Project Most Valuable Professional – MS Project