Ok, this is how I converted my spear chukkaz.

What'll need....

Spear Chukka 1:

Empire morta back. (The tail part that you don't use if you're making a cannon.)
2 Goblin shields (The rectangle ones with a spike.)
1 Flag bone (The bone from a night goblin command sprue.)
1 Goblin flag (The one with dangly things...)
1 Orc Spear (Doesn't matter which one, you'll probably want to get rid of the arm. You only need the top part of it, so after you've taken off the top end you can toss it back into your BITZ box.)

Ok, first of glue the 2 shields onto the front of the morta back, then glue flag bone behind them with the notch facing up, then take the orc spear and glue it the notch, then take the goblin flag and cut off two of the dangling things then glue them on the flag bone so it looks like they are the ropes.

I'll post pictures as soon as I've painted it.

Spear Chukka 2:

The actual morta
1 Orc spear (Same as last time.)
2 spiky shoulder pads from the orcs (The thinner one.)
1 Empire shield
1 Orc spike (The spike from the orc sprue)
2 Goblin shields (The rectangle ones with skulls.)

Ok, this one needs a little cutting, cut a whole in the inside of the morta only big enough for orc spear to go into, then glue the 2 goblin shields on the back of the mortar, then glue the empire shield at the bottom of the front, it should look something like a trypod now, then glue the spike on the whole on the top ofd the mortar, then glue the 2 spicky things on the sides so it looks more gobliny.

Pictures when I've painted it.

Also converting goblin crew is a snap, take some extras goblin legs (If you have any from wolf riders which you should!!!&#33 and look threw your bits box for any left over bodys, (If you do 40k the dark eldar body's from the old starter set actualy look good with some cutting and some added bitz here and there....)