It's often necessary to add a domain to Blesta that's already registered and in your account. In this case, we follow the following procedure (successfully with Generic Domains), but the domain is missing, there are no service_fields that match our domain's service ID after we are done, but the domain should be present with the key=domain AND service_id= service ID.

1. Install the Namecheap module.
2. Add a TLD, like .xyz using Namecheap. Packages > Domain Options: TLDs Pricing.
3. Visit a client profile page, click [+] in the domains widget. Enter an existing domain, like domain.xyz and click Transfer. Uncheck "Provision the domain using the registrar module". Enter a fake EPP code, click continue.
4. Note that after this process, the domain is added but there is no label, so the domain appears blank. Editing the domain as a service likewise does not show the domain, nor a field where it could be entered.

Repeating this process with the Generic Domains module, and Namesilo, and perhaps all other modules (not all have been tested), this works just fine, the domain appears in the label.