Hi,

Do you already have a working project using TI Tiva/Stellaris without a BSP? If not, then that should be the first step.

The downloaded BSPs are stored in %LOCALAPPDATA%VisualGDBEmbeddedBSPs, you can edit and add your own BSPs there.
Have a look at how some of the BSPs there are made, try using AT91SAM7S or STM32 as a reference as TI Tiva/Stellaris should also use the arm-eabi toolchain. Try initially only defining one MCU in the BSP.xml, once that works you can add others based on it. Make the BSP.xml data based on the TI Tiva/Stellaris datasheet and add the peripherals, startup file, linker script in the directory tree. Those files should be available from the TI website software for Tiva/Stellaris, the peripheral library is usually a separate download and the startup/linker file is usually part of an example. Finally add a basic sample to make new project making easy.

Let us know if you need any clarifications.