Convert a png to dds

[anthony]

It means it can read the format, but not write it.

If you want Im to write to it you either need to tell image about a external image converter it can use.
See http://www.imagemagick.org/Usage/files/#delegates
OR you need to get some programmer to add write capability to the DDS coder module in the source.

The former is easy, and is done by having IM write the image as one format in a temporary file, then it runs the delegate program to convert. But it is a stop gap measure that may miss information that IM has access to but the intermediate format may not save, such as profiles, comments, labels, image layer offsets, etc.