There isn't much that is not possible, given sufficient time and money.
When I bought my GS the PO assured me he had just had the shock rebuilt, but if he had it was a waste of money, as the performance was mediocre at best.
For not a lot more you could probably get a basic, rebuilable, Wilbers or YSS, and if you buy it from a BMW specialist it will have the correct spring/ damping for the bike, and you wont need the multi adjustability which jacks the price up considerably.
Remember all shocks need a fluid change/regas/ service at regular intervals , so buying a shock with a good spread of dealers who do it at a sensible price is important - I get my Ohlins serviced anywhere for 45 quid but the sole Wilbers agent wants100 quid to service the Wilbers.
if you can wait second hand Ohlins sometimes come up on Ebay, and you will never regret spending the extra to get the best!