how to add an author whose proper name includes a ","?

I would like to add "God Is Still Speaking,®: The Writers’ Group" as an alternate name for Stillspeaking Writers' Group. (See https://www.ucc.org/stillspeaking-writers/ if you wonder about the validity of this desire 🙂 .) The comma is actually a meaningful part of the name.

Is there a way to get LT to interpret the comma as not the divisor between "Lastname, Firstname"?

Nope… comma is always considered a separator.

I’d add a disambig note in such cases, explaining the situation and that the comma is meaningful.