Mapper has the ability to process large data sets as sub-projects which can be merged. Fields struggles to process large data sets for me , 500,000 images, but when processed as sub-projects or separate flights I am unable to merge the consequent layers. Can I import a project between the Mapper and Fields software or am I to just wait for the feature to become available in fields?
this is currently not possible in Fields and also not on the roadmap so far.
I suggest you use the merge and or mosaicing tools in Qgis to put all the rasters together after you processed them in fields.