David,
I checked Outlook 2003 and 2010 this night

and fund in both programs there is an option to import digital ID's. Even Outlook Express 6 has that option. Outlook Express only accept files in the following formats: Either *.SST (Microsoft Serial Certificate), *.P7B (PKCS #7 Certifikat) and *.PFX / *.P12 (PKCS #12 Certificate). Outlook 2003 accepts only *.PFX, *.P12 or *.EPF (whatever that is) while Outlook 2010 only accepts *.PFX or *.P12.
1. How to import in
Outlook Express (if anyone is dumb enough to use this

): Go to Extras - Options - Security. There you can see a passage "Secure E-Mail" (bottom of that tab), there is a button "Digital IDs", if you click you can see all certificates which are in use and you have a button "Import...".
2. How to import in
Outlook 2003: Go to Extras - Options - Security. There at the bottom should be a passage §Digital IDs (Certificates)" and a button "Import/Export...". With that you can browse for any file with the appropriate extension to import. You need to give a name to the certificate but thats all.
3. How to import in
Outlook 2010: Its somewhat different

. Go to File - Options - Security Center. Klick button "Change settings for security center". Choose "E-Mail security", there you find a passage called "Digital IDs (Certificates)" and a button "Import/Export...", the following steps are similar to Outlook 2003.
The only thing I didn't find out is why M$ only uses this *.PFX/*P12 format and nothing more...
Arioch
Ummm... I hope I have translated my German menues into the right English button- and menu-titles. As I am using only German Office I can't change the language in the programs, only in the spellcheck...

Armageddon has been yesterday - today we are in REAL trouble...