There are two ways to do it.
First means finding a power steering rack for the Express. This will be the stumbling point of this option. If you can find a rack, then the easiest way of powering it is by using an electric stand-alone pump from something like a Saxo (from your local breakers yard). Find a yard where you remove stuff yourself, that way you can remove the loom and pipes. This will greatly simplify engineering it into the Express.
Option two involves getting an electric steering column (from a Corsa). The standard Express steering column will need removing and some mods will be required to make the Vauxhall coulmn fit and look neat. A control module will be needed for the columns electronics, these are about �40 on Ebay. This is probably the more viable option given the rarity of you vehicle and does away with the need for a new rack. Price wise, you could possibly do this yourself for not much more than �100 with careful buying.
Both of these methods have been in use for many years in the motorsport /kitcar world and a few searches on Google would, I'm sure, unlock loads more info.