Currently, it is only possible to transfer pension accounts held at UK pension funds. There are a lot of expats in London that have worked and contributed to pension plans outside the UK; it would be nice to be able to regroup all that into FreeTrade SIPP.
There’s seems to be a lot of paperwork involved I’m not surprised they don’t offer it.
You could do it indirectly transferring to a uk pension provider which supports overseas pension transfers (likely at a cost). But it also depends on a number of factors. You pension needs to be a recognised overseas pension, you need to understand any tax implications, you also almost certainly need to speak to a financial advisor before you consider any actions as you could be worse off transferring than keeping it where it is.