If the custom model has its own skeleton, then you can sometimes make better adjustments to fit the custom mesh to the original survivor mesh than if there weren't a skeleton.
Move the custom model's mesh with its own skeleton using Pose Mode.
In Mesh's Modifier panel, in Armature Modifier, click Apply button. (Makes the mesh stay with the skeleton and makes the modifier disappear.)
Select the skeleton pose.
Select: Pose Mode
Select: Pose > Apply > Apply Pose as Rest Pose
In Mesh's Modifier panel, select: Deform > Armature
Click the Object field.
Select the armature from the list.
Note that the vertex groups and their weights are separate from armature and stay unchanged.