I can understand what you're saying with this suggestion, however adding them to the donator store wouldn't be the way to go - in my opinion.
On the other hand for example, if someone owns 2 helms and a body then the option to trade 1 helm for 1 legs might be acceptable. (I think in a controlled manner, with approval before hand maybe?)
An obvious issue may be that if someone has a large quantity of a certain piece that they can then get multiple sets - this shouldn't be the case hence the controlled manner point above.
I mean maybe it would be much better for the economy if there were completed sets in-game rather than a lot of mis-match spare parts.