The background is that we have a prebuilt object file without unwind table, but somehow gcc unwind had problem backtracking on the object. Is it possible to generate unwind table without source code? Considering unwind table is based stack statics which is also available even without source code.
In general, it is not possible to generate proper unwind tables from machine code in an object file. For a start, some constructs are quite difficult to represent accurately in unwinding information. Retpolines are an example.
The larger practical problem is that DWARF unwinding information is structured per function. A bare object file (without debugging information and only a minimal symbol table) does not capture function boundary information. Without that, it is impossible to say if a location in the file is the target of a function call and the start of a function. Similarly, a call to a noreturn function may be the last instruction in a function, even though it is not followed by a return instruction. It may be possible to use relocation data. There are several tools out there which attempt to infer function boundaries; every disassembler does it to some extent.
Your best bet is to locate the functions which fail unwinding and figure out why, and then compensate for that, either using custom-written unwind data or a GDB plugin. As Alexey Frunze said, a full conversion will be rather tedious.