Sure, here are the instructions for adding custom blocks:
We don't offer a set of movement blocks as standard since there are so many different possible variants. Some applications want north/south/east/west movement, some want up/down/left/right movement, some want forward/backward/left/right movement, some want absolute x/y movement, some want relative x/y movement. And within each group, some want constrained magnitudes (e.g. 1 step, 90 degrees), whereas others want unconstrained magnitudes (e.g. 4.2 steps, 22 degrees). Some applications want computed magnitudes, others want simple built-in fields.
This is why we don't provide turtle blocks, but instead allow developers to create their own. For reference, you might want to look at Blockly Games Turtle:
Note that even here there's a change in that levels 1-9 have constrained magnitudes, whereas level 10 has unconstrained computed magnitudes.