I have a question about how to convert a segmented hippocampal subfield file into a ROI.

After -hippo-subfields processing using recon-all command, I have got several files named posterior_left/right_**.mgz. And the segmented volume just coverd a small area in subject's whole brain volume. Now i want to convert the segented volume into subject's whole brain volume space and I can use it as a ROI. Is there any solution to do that?