You can use exactly the same algorithms for going to ARGB8888. There isn't any alpha information in the YUV, so you just need to set the alpa channel to 0xFF for every single pixel. If you leave it at zero then you will get a blank image because that means 100% transparent.
Going from ARGB8888 the most obvious option is to ignore the alpha channel.
The other solution is to pre-multiply all color values by the alpha value and divide by 0xFF. This has the effect of not changing fully opaque pixels, making semi-transparent pixels proportionately less bright and making fully transparent pixels black. This is probably not what you want but works for some odd cases.