Unless some licensing issue gets in the way (which I don't see there being
any since you can write add-ins with a plain text editor), you should be
able to no problem.  Add-ins are just assemblies written a specific way
(Implementing specific interfaces) and then building.  There is nothing
"special" about them.  You may have a problem testing and debugging them in
the express versions though.....

HTH,
Mythran