Thanks for the clarification! Yes, you can copy the backup files but keep in mind: you have to copy the whole backup chain including .vbk, .vbm and .vib files for further incremental restores or .vbk and .vbm for restoring the full backup restore point only.
Copied backup chain is no longer affected by retention and you have to delete the retired backups manually. It might be tricky because deleting the incremental backup in the middle of the backup chain makes all the further incremental restore points corrupted. Additionally, do not forget about the automatic merge of the retired incremental restore point into full backup: How retention works
With these being said you either should set a long retention coping offsite only newly created increments or short retention (2-3 days) with complete backup folder sync.