It is not possible to use Salsa's Advocacy platform to allow supporters to submit comments to sites like regulation.gov. There are no plans to implement this.
That said, typically there's an alternative submission option that doesn't involve the form. For example:
"For alternatives to on-line submissions, please contact Jane Doe, Policy Staff Committee, at (555) 555-5555"
When provided with an alternative contact method, such as the above, you might consider collecting letters from constituents using a black holed target, then email the exported PDFs to them (if they accept submission via email). Reaching out to that contact might provide the email.