I think it can be done, slot 2 xeons are basically nothing more than PIIs or PIIIs with extra cache (some are even with the same amount of cache). It seems that slot 1 to slot 2 adapters exists:
https://www.anandtech.com/show/318
Slot 2 to slot 1 could be done, power requirement might be a problem, as nforce4max said, but they could also be solved by attaching a molex connector to the adapter and converting 5 and or 12 V to the required voltage for the cache.
However, the fact that such an adapter doesn't seem to exist might indicate that there might be some other problems. Or there would be too expensive. Or there wouldn't be a demand for such product. Back then (in 98-99) a Xeon CPU was very expensive, and it was used in servers in dual/quad configurations. Pairing such an expensive CPU with a mainstream single slot 1 board didn't made sense back then. But now it would be fun to try.