If you want to offload SSL to HAProxy on pfSense and let ACME handle the validation, you can. You do not need to import anything from your other servers, let the ACME package create and request the certificates for you. Even though they already exist, as long as the validation passes it will be OK.
From the look of that last error you do not have the proper settings for DNS validation.
You could instead follow the example at https://forum.netgate.com/topic/90643/let-s-encypt-support/31 and configure it so HAProxy can assist in handling the validation instead of using DNS.