Hi Falcon Drones,
At the moment you cannot do this process directly in Pix4Dfields, it is a feature we are looking to bring to a version in the near future.
In the mean time you can benefit from the fast and accurate processing of Pix4Dfields and then export your orthomosaic (and/or index maps) as geoTIFF files. These multiband images can be split to individual bands using an open source free tool such as GDAL on the command line. Note, depending on the tip of images being imported we include the alpha band to produce the transparent background between the edge of the orthomosaic and edge of the rectangle. I’d advise therefore that you would export the single band plus the alpha to retain a transparent background but this is your choice.
Information about how you can do this process can be found here: https://gis.stackexchange.com/questions/62133/how-to-export-only-one-band-from-an-image-using-gdal
If you need further information please don’t hesitate to write on the community.