To do this semi-securly (because slack accepts regular passwords) you'd need validate the user's own mfa before handing out these mfa creds to prevent a slack account compromise from escalating... but slack can't do that unless there was an extension in the plugin somehow to prompt for an otp code.
Would recommend hashi vault in place of this which can work for human/non-human workflows and a pretty robust security model with better auditing and authorization than slack. https://www.vaultproject.io/docs/secrets/totp