Mario, yes you can. One of our users Michael Koontz had attempted to merge and it was successful. You can see the comments here: https://community.pix4d.com/t/7773–Desktop-Cloud-Hybrid-Pix4Dmapper-Pro-4-3-31-imagery-with-different-spectral-signatures-one-block-after-subproject-merge-two-blocks-after-cloud-processing
It will be the same as merging two projects together (https://support.pix4d.com/hc/en-us/articles/202560689-How-to-merge-Images-with-different-Spectral-Signatures-RGB-NIRGB-etc)
If you have any issues do not hesitate to reach out 