There's some API functions that let you to modify the Registry, like RegOpenKeyEx, RegQueryValueEx, RegCloseKey and RegEnumValue. What I need was to access to the HKEY_CLASSES_ROOT, where you find the file types. I couldn't access to this part of the Registry through these API, even using the constant HKEY_CLASSES_ROOT = &H80000000. So, I have to think another way of adding data to the Registry.

When you want to export some data from the Registry, the application Registry Editor have an option that makes a file with the extension .REG, and saves all the tree you want to a text file.

My program creates a .REG file, and then executes it with the Registry Editor to add the information.

Open App.Path & "\RG.reg" For Output As #1
Print #1, "REGEDIT4"
Print #1, ""

'tExt is the variable extension, like .VBH
Print #1, "[HKEY_CLASSES_ROOT\." & tExt & "]" 

'@ means the default data for the Registry Editor
Print #1, "@=" & Chr(34) & tExt & "file" & Chr(34) 
Print #1, ""

'an extension in the Registry is divided in 2 keys
Print #1, "[HKEY_CLASSES_ROOT\" & tExt & "file]" 

'Chr(34)=quotation marks, because it's String data
Print #1, "@=" & Chr(34) & T & Chr(34) 

Print #1, ""
Print #1, "[HKEY_CLASSES_ROOT\" & tExt & "file\DefaultIcon]"
Print #1, "@=" & Chr(34) & _
    "C:\\Windows\\System\\Shell32.dll,1" & Chr(34)

Close#1

Note: You can use this technique, not only for HKEY_CLASSES_ROOT, and not only for String data. If you have questions, you can extract from the Registry some information to a file, and then see the file to know how is this "language".