This is definately not the best way to use an MSC-400 but here is what could be done.

EDIT:
The only reason I say this is not the best way is because this method does not use the narrow band remotes or the narrow band RFX-250 and some could end up with RF issues they would not have if using the narrow band remotes and RFX-250.
END EDIT:

There is no problem using the RFX-150 with an MSC-400 if you don't have RF issues, the non-narrow band remotes should work fine with the RFX-150.

In a normal MSC-400 system you would program the remote with devices, but no macros, then import the devices in the MSC editor. You would then build the macros in the MSC, then re-open the remote file and use the universal browser to drag the macro triggers into the remote. The MX-700, 800, and 850 universal browser will NOT open a MSC file (even though it says MSC, it is looking for a .msf file not .msc) there is no intent at this pooint to include these remotes because they currently are not narrow band remotes (although they may be in the future).

The trick now is that if you have a remote that you can drag these triggers into, like your 3000, you can set up that remote with the triggers you want to use from your other remotes. Set the 3000 to RF and the correct ID and test the triggers with the MSC-400. If they work, set that device to IR and use that remote to teach your MX-350s those triggers as IR commands. Set the MX-350 to send those commands RF and set the right ID. Now the MX-350 can send these triggers to the MSC-400.

I do this to do an ALL OFF macro for my living room system, from my MX-800 in the bedroom. As a side note my MX-800 does work with my RFX-250 but there are no guarantees that will work for others.

This method can also be used with hard wired keypads that can be wired to the IR in on the MSC and a keypad can trigger the macros in the MSC.