This is a dangerous technique but this does not stop the Skaven in their hunger for power.
See the rules for Warpstone tokens in the section on Skaven magic.

In a similar way to scrolls, all Skaven wizards in the army can buy as many Warpstone tokens as they can afford within the limit of the points they can spend on magic items. Buying one or more tokens does not count as taking an Arcane item.