For converting a .h file to a form designer file, you will have to make changes in the .vproj file.
Example: - If we have a Login.h & Login.cpp files (The .cpp file should have InitializeComponent() function definition) and need to convert the Login.h file to Login.h[Design](For editing the Gui).
Search for Form.h file entry in the .vproj.
<File RelativePath=".\MyProject\GUI\Form.h" > </File>
Change the file type to "FileType = 3" and also add entry of its corresponding .resx file to the block.
<File
RelativePath=".\MyProject\GUI\Form.h"
FileType="3"
>
<File
RelativePath=".\MyProject\GUI\Form.resx"
SubType="Designer"
>
</File>
</File>
Save the changes in .vproj and reload the project. This will convert simple.h file to a designer file.
Regards
Kedar