Hi,
materials point to their directX LPD3DXEFFECT struct. It has many methods to modify the static table of the effect.

Code:
LPD3DXEFFECT _fx = _mtl->d3deffect;
if (_fx == NULL)
   error("unable to retrieve the effect of a material");
float _f[4] = ...;
if(_fx->SetVector("customName", _f) != NULL) // see: SetFloat, SetVectorArray, SetMatrix, SetMatrixArray...
   error("unable to modify the static table of an effect");


https://docs.microsoft.com/en-us/windows/desktop/direct3d9/id3dxbaseeffect

Salud!