Yes. If you import a listing with the same MLS number of a listing that you already manually added, the listing data from the MLS will be imported and merge with the manually added record. The data imported from the MLS will override any manually added data if it is different. Ex. If you manually added the sqft. as 2,100 and in the MLS it says 2,150, the value will update to 2,150.
*Merging records does not affect any offers received or settings on the original record. Any settings or offers from the original record will remain in the merged record.