As already mentioned by Vinny, to change status from Tier 1 (General) migrant to that of dependant of Tier 1 (General) migrant, an application (for entry clearance) must be made from outside the UK.
If the applicant meets the requirements, and submits the necessary documentary evidence(s) to support the application, there is no reason why a visa will not be granted.
See also
Points-based system Tier 1, General Migrant - visa application guide, and for queries regarding Tier 1 (General) please post in
this forum.
regards