It's the case with any pension that you're already receiving. You can't pay that directly into another pension scheme and receive tax relief on the contributions as they're not classed as earnings. If you think about it, that's fair enough, as you've already received tax relief on those contributions once, so you would be receiving it again if that was allowed.
Obviously, if you're still working you can increase the pension contributions from your earnings by an equivalent amount. However, that's where the rules about recycling come into play, as you might be using money which has already received tax relief (i.e. the tax free lump sum) to fund those contributions.
It sounds like Teesste and Massimo's Header know more about the detail of how HMRC considers recycling than I do, but I thought I should point out that this is one of those areas where it's really advisable to speak to someone who actually knows what they're talking about.
Just seems a bit silly to be forced to take a pension, when you don't need it, so end up paying more tax when you don't need it, rather than just being able to transfer it to another, for when you do. I can understand if you're forced to take it or got the money from elsewhere, but when you don't have to take it, you should be able to port it, without ever seeing it.
HMRC are basically putting a tax block on people protecting themselves (yet they say they want people to prepare better for later life?), so people will get it, spend it and then have less for later in life, so they'll end up taking more from the state.
Personally I'd just increase pension contributions from work, to the same amount. It's being earned, so can go straight in the pension pot, and the pension is paying out which enables the contributions. It's silly, to have to do it, but I would, plead ignorance, and then fight them if they ever kicked off about it. I'd speak to my FA first mind.
Putting money into a pension is meant to get relief, this is meant to be a guarantee/ benefit, not a hinderance, and should not need to be taxed when it comes out the first time and then second (once is enough), as it completely does away with the actual benefit of it being tax deductible. Yes, they could argue that you benefit of compound interest, but pensions still carry risk, like any investment, and could argue against that anyway with opportunity cost. The government also benefits from the compound interest too mind, why would they want to take 20% of 100k, when they can take 20% of 150k, and then 20% again for the second time.