BuddyPress.org

You can manually add a member avatar to your avatar directory.

eg.
example.com/wp-content/uploads/avatars/USER_ID/hldkfhlaskjhdfljsdfdf-bpfull.jpg
example.com/wp-content/uploads/avatars/USER_ID/hldkfhlaskjhdfljsdfdf-bpthumb.jpg

The suffixes are important as BP will pick up any image with the “-bpfull.jpg” suffix or the “-bpthumb.jpg” suffix.

If you only have to do this once, then this is sufficient.

If you want to replace all the member avatars with a default one, try reading this:
https://buddypress.org/community/groups/how-to-and-troubleshooting/forum/topic/group-avatars-how-to-change-the-default/#post-45764