I don´t know if this work flow works in FC Express, but this is what I do when I want SD footage in FC.
Shoot in any HD format, drop the footage on a HD timeline, open a new SD timeline (DV, whatever you want).
Copy all the footage from the HD timeline and drop it into the other SD timeline and render it out.
If FC Express can´t have multiple timelines open, make a SD timeline and drop the HD footage onto the timeline and just click "NO" when FC asks if it should change the timeline setting to match the clip. Render.
I understatnd this don´t answer your problem right now, but for the future, just shoot HD and let FC do the down conversion for you.
This way you´ll have the master clips in HD if you need it one day
EDIT: I´m sorry if this workflow for some reason dosen´t work in Express, I only ever used Pro