Unfortunately, this is not possible. To my knowledge, each resource references one palette. There are a few ways around this problem - you could create a multi-layered background so that you can use more than one palette, you could over-write the palette in-between blanks or you could display your high-quality background as a raster instead. Extended palettes refer to palettes external to the standard 256-colour [BG_PALETTE] one that refers to the whole screen, however each resource in tiled mode still refers to one 256 colours palette and going beyond that limit is quite the task. Personally I found out that when converting resources, the decrease in quality highly depends on the software used. Do try Paint.Net and IrfanView - they're both pretty efficient at saving images with 8-bit palettes without a substantial decrease in quality.