No, you cannot add your own money to the voucher for tax reasons. It's best to choose your bike prior to applying for the voucher so you know exactly how much you'll need. A number of shops will take Halfords vouchers directly but I only know of the London ones. I got my bike directly from Pearson in Sutton. I got a bit off the accessories so that the bike and bits came to £500 on the nose and I didn't have to get a bigger voucher (our company only allows you to jump by £50 at a time).
My Halfords voucher came with an extra 10%, redeemable at Halfords only. I thought that I had forgone this by purchasing at another store, but the other day I got a voucher for £50 through the post so I can buy one or two more accessories. Not sure if this is standard for the Halfords scheme or just something my organisation has negotiated but it's not bad as you don't have to pay this bit back.