Whilst a tad non compliant if you found a shop that was willing to let you buy the conversion kit that is on the scheme you could get it passed through. I take this snippet from cyclescheme rules:
I already own a bike; can I obtain just safety equipment and accessories through Cyclescheme?
Yes. Your Cyclescheme application can include safety equipment and accessories without the need to include a bike in your request.
If you were to find a shop, obtain the value you need to claim for, say £800, then you just submit through your employer a list of accessories you make up for £800 that are allowed on the list, i.e. helmets, clothes, locks, panniers bla bla. No one knows what happens between you and the shop. This is how some people even get two bikes. They submit for a £1k voucher and walk out the shop with two £500 bikes. That is entirely down to the shop / customer relationship though and nothing to do with what gets submitted through the employer.