Orks generally come with a very boring pose. Here is a simple trick to give your slugga boys more WAAARGH !
I cut off the rear leg as near to the groin as possible.
I rotated it and glued it back in place. Its a big joint so you don't need to pin it.
Then I filed it round and filled with just a very little green stuff.
To make the running pose look "real", you should pick arms that make the figure look unbalanced. There a a few arms that you can use straight out of the box, otherwise you will need to convert again.