To render out zpic files, first you need a Main channel that outputs pic files, and then you have to add a Depth channel and change its output format from zt to zpic. Here’s a short video that shows how.

Here’s the step-by-step:

1. In the Render Manager, add a Depth channel (under Render Channels Output, click Add).
2. In the Create Framebuffer From Render Channel dialog box, click Depth in the Render Channel list, and then click OK.
3. By default, the output format of the new depth channel is zt.
4. Select the new depth channel (click that row) and then click Edit.
5. In the Output Format list, click Softimage Zpic.

See the wiki page Zpics and the FX Tree for more info zpic files.