Hi,
We did the same as Shashi mentions. My customer uses a subset of UNSPSC (3k in all), but he wants to show to the buyers only 90. So we created those 90 as Custom Commodity Codes, and then mapped the remaining 2910 to any of those 90 (plus a 1-to-1 map for those 90, too) from UNSPSC to CUSTOM.
If you have 22k, it is the same, except that the mapping is gonna be way bigger, but the concept is the same.
Hope that helps.