Since I now have the red slashed SIM on my N900 my wife has offered up her N900 and a temporary move to a Nokia dumb phone while she waits for a One+ phone.
There is an easy to find option to import contacts from the SIM in contacts on the N900 but I am not finding where to export her contacts to the SIM, am I missing something?
Unfortunately the dumb phone has no bluetooth or even IR so no easy way to jump them over except on the SIM or manually enter them.

I do not know any tool which can do that. I even do not know if Maemo sscd/csd/... daemons or libsms/libcall/... libraries exports some API for it. Jonwil should know more if it is possible via some Maemo (dbus) API.

But I think it should be possible at least via sending ISI packets (direct communication with modem). Some documentation still exists, see: http://talk.maemo.org/showthread.php?t=94275

I wonder if something like that could be possible by exporting the contacts into a big vCard file and then use some kind of SIM reader/writer and perhaps a script to save it to a SIM card.

IIRC notebook's smart card reader can be used to modify contacts or SMSs on SIM card

hmmm, I am now on the road or in various hotels for about a week. I wonder if I can write up some sort of usb to SIM bit-bangy interface with a little digispark(attiny85) and some snipped paperclips for contacts; not sure I have anything along to do that.
I will probably read the vcard fies and just plug them in by hand.
I had forgotten the N900 could even read from a SIM, was dissapointed we never got write working.