Sure, you can always click the “select a third-party toolchain” button to import an arbitrary toolchain. VisualGDB will create a Toolchain.xml/CustomToolchain.xml file either in the toolchain directory or under %LOCALAPPDATA%, where it will store the toolchain properties. You can then manually edit the file to customize various settings (e.g. add hardcoded CFLAGS). See this page for a detailed explanation.