Combining UE4 created Shader Material with a second diffuse map

Hey guys,
I’m wondering how can i blend on a UE4 created shader / Material like “Metal rust combination” an additonal diffuse map.
For example i have a metal balk, the material / shader is fully created within UE4 but i would like to add some details to it, so basically blend something over
the original material.

The second material is a simple glass material, same case, i would simply like to blend a diffuse map over the glas material without loosing the transparency effect from the glass.
i tryed to add the second diffuse map by multipy it with the cube map in the diffuse channel, i aso tryed to use a lerp for combine thoose two maps but it didn’t worked out so well…