Here are the two paintbar files. Add these two underneath your RSI and link them together. One is for upper BB line, the other is for lower BB line. You can change the parameters in the paintbars to modify the Bollinger Bands. Make sure to make the paintbar widths minimal (2) and make the shape something like a triangle so that they don't interfere in drawing the lines.
This is not the best or most elegant solution. Paintbars are really not fully suitable to create new indicators. But the custom indicator editor is not there yet.