Archiving is one way to slim down your sync database, but I don't think you'll want to store these items in there. (More info on archiving is available
here, just in case that helps.)
I'd suggest you try the following, instead.
Quit OmniFocus, then open up <your home folder>/Library/Application Support/OmniFocus in the Finder.
Select the file named "OmniFocus" (or "OmniFocus.ofocus").
Select File -> Duplicate from the Menu bar. Change the name of the new file from "OmniFocus Copy.ofocus" to "Someday-Maybe.ofocus".
Delete everything
except the Someday/Maybe actions from the new database, then move it some location that's convenient to double-click. You can now open the second database as you need it. (To add stuff, to move things to the "real" database, or what have you.)
You can now slim down your "real" database by removing those someday/maybe actions.
Does that help?