I had a good hard look into my Praxis and found a possible way to upgrade it to slamfire.
Only the part I call lockplate (because its only function is to block the release if a lock is set) needs to be replaced for that.

This is work in progress, I havn't got it to work right now. The mod changes the function of the trigger from pushing the lock plate to release the spring, to blocking a secondary new spring (rubber band, etc.) from forcing the lock plate to release. This, paired with a new pusher extension which returns the lockplate to its non-released state via the pump action mechanism should allow the gunner to just hold the trigger and load and release shots just by pumping. The balance between the trigger and this release spring need to be carefully balanced....

On the parts picture you can see a few iterations of my part, together with the original one.

If you get it right, plz share! =D

Oh, disclaimer: I did not laser scan the original part, I reengineered it in Sketchup8 (I really need to learn a better tool).
And: If you break your Praxis, hurt yourself, make baby jesus cry... Sorry, not my responsibility.