Rigi-All

This is a free tool that speeds up adding Rigify bone types to existing deform armatures. You select appropriate bone chains, then use a custom side window to pick appropriate bone types. Can automatically rename bone chains to be symmetrical based on user configuration. Also has an option to add a DEF- prefix to vertex groups of a selected mesh.

Compatible with 4.0 and up starting with version 1.2, older versions compatible with 3.6.

Tutorial by the addon creator