That's not just a DCM thing, but happens in SPM more generally. The SPM authors were (reasonably) of the opinion that data extraction in the results context should be done using suprathreshold voxels only.
If you want to get around this, either because you are determined to use precise coords that aren't suprathreshold, or because of a paucity of suprathreshold voxels, I don't know of any way to do it other than playing with the SPM code, unfortunately.
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