@dbsim4 first to the second question:
If you manage to import the voxels in Sim4Life, you can selected the LabelField entity and run the Image Tools / Generate Surfaces tool. I would recommend to turn on smoothing and simplification (edge length typically >= voxel size, depending on the feature size you want to preserve).
To import the voxels in Sim4Life, you should write the voxels as a binary file (8-bit, 16-bit or 32-bit unsigned integer). A quick google search resulted in this MATLAB documentation page on writing binary data: https://ch.mathworks.com/help/matlab/ref/fwrite.html
Alternatively, you could find a package to export the voxels as nifti file, and import the nifti file As LabelField (option in import dialog). The importer can also load an iseg tissue list, which assigns colors and names to the tissue labels. An iseg tissue list contains following information. First line is version, second line contains number of tissues not including background with label=0. The subsequent lines start with C and list the color (RGB-Alpha) and name.
Example:
V7
N3
C0.00 0.00 1.00 0.50 Bone
C0.00 1.00 0.00 0.50 Fat
C1.00 0.00 0.00 0.50 Skin