Yes, you can send a kudos to anyone who is in your Slack workspace. This includes non-Matter Slack channel members.

Kudos sent to team members who are not in the Matter-connected Slack channel will receive the kudos, and if the Slack channel is public, will be automatically added to the Slack channel.

If the Matter-connected Slack channel is private, then you'll need to first add the team member to the Slack channel and then send the kudos.