There is no support for WMS at this time. We have looked into it, but since most WMS sources are not in the web mercator projection that we use within ReefMaster, it would take a substantial amount of work to implement (we would likely have to introduce variable projections within RM to accommodate this).
We do have support for user-configured WMTS and Http tile map sources coming in the next maintenance release, but only for sources in web mercator/ESPG 3857.
There are some tools out there that you can use to convert WMS raster data to the MBTiles format, which could then be used as a background image within RM - this might be worth looking into.