You can't explicitly add symbols to the index. Instead, you can create a tmPreferences file with the appropriate scope. Here is the entry that is built into ST.
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
These files are merged together, so no need to include "entity.name.function, entity.name.type" elsewhere. To view indexed symbols, take a look at window.lookup_symbol_in_index() and window.lookup_symbol_in_open_files().