This is interesting. XEP-0004 does not define any extensions in Data Forms, nor does XEP-0050.
The only place I saw it is in Example 32 of XEP-0133, but... it's just an example without a real specification.
There are some defined extensions for Data Forms, but they are all well specified in their own XEP. (e.g. XEP-0122, XEP-0221)...
Therefore I am just wondering about this issue.