@Rackscha
Yes I will release it for free, but give me some time to solve the iusses with it.
I worked on it for a week. It is not too complex code, the tricky part is to reduce the memory usage.

@MasterQ32
Ha, didn't saw that. Does the welding has a tolerance?

@Bone
For the ease of use I use the function in a DLL for now, but I think you can just add it to the d3d9.h header file. Don't forget to implement the used structs too.
function:
msdn - link

edit:
D3DXWeldVertices(LPD3DXMESH pMesh, DWORD Flags, D3DXWELDEPSILONS *pEpsilons, DWORD *pAdjacencyIn, DWORD *pAdjacencyOut,DWORD *pFaceRemap, LPD3DXBUFFER *ppVertexRemap);