please excuse this reply coz it's a 'coming home drunk from a party reply' but I think you can use Cults of Prax and all the other RQ2 supplements in your games because they were all cool. Use common sense for the rules side of it (which, after all, doesn't matter as much as the gloranthan experience/story side) and just go with it....
let's face it, storm tribe's version of urox (storm bull in CoP) is bound to differ from the Praxian cult.
IMO, don't worry to much about the conversion aspect, write your story paying respect to the myths and stories that make glorantha for you.
again apologies for drunken directness but I've found that running a campaign in glorantha these days (using HQ) is far better than is used to be - focus on story and not whether Stormbull gets Earth Power anymore :)
here's wishing you well